For months, city and union officials have been expressing optimism about reaching a deal on new teacher evaluations by a state deadline in January — with some road bumps, of course. But what is keeping the two sides from reaching an agreement has not been clear.

That has started to change in the last week, as Department of Education officials have spoken publicly on multiple occasions about sticky issues that are still being worked out. The issues include how often observations should take place, what the observations should focus on, and when to schedule hearings of teachers who want to appeal low ratings.

Union officials have declined to comment on open issues, saying that they did not want to discuss negotiations while they are ongoing. But a top official said that no issue would be considered fully closed until the entire evaluation system is set.

David Weiner, the deputy chancellor in charge of teacher quality, stressed that the issues were “not sticking points” when he spoke with teachers at an event last week hosted by the advocacy group Educators 4 Excellence, which supports new evaluations. Department officials made the same assurance Wednesday morning after a panel discussion about teacher evaluations held at the Manhattan Institute, the politically conservative think thank.

Instead, they said, the issues are simply very complicated to resolve.

Though some of the evaluation system is set by state law, the city and the teachers union must negotiate many major points. For example, state law says that 60 percent of teachers’ ratings must be based on “subjective measures,” with at least half of that going to observations by principals. But exactly how the 60 percent is broken down and how the observations should be structured is up for discussion.

The city and the union also have yet to agree on several components of the observation process, including how often observations should take place and the procedure required to document them, city officials said Wednesday.

Currently, a single formal observation is used to generate teachers’ ratings, and the process must include a pre-observation, where the teacher and observer set expectations, and a post-observation to discuss what happened during the observed lesson.

Now, the state is requiring that principals observe each teacher multiple times, at least once without letting him know ahead of time. Weiner said last week that the parameter left much open to debate.

“We’re trying to figure out what seems appropriate,” he told the teachers, before launching into a litany of options. “Should you have five a year of that pre-observation, observation, post-observation? Should there be two pre-observation, observation, post-observation? Should we eliminate the pre-observation and the post-observation and just make it an ongoing cycle of unannounced visits? Should the observations be shorter — should [observers] come in for 15 minutes at a time?”

City and union officials would not say what they hoped would be included in the final agreement. But Micah Lasher, a former top Bloomberg lobbyist who is now pushing for new evaluations as the head of StudentsFirstNY, said last week that their broad positions are easily predictable.

“Generally speaking, the position of the administration is going to be, you know, they want to give principals as much latitude to do more observations with less bureaucracy,” Lasher said. “And I think generally speaking the position of the union is going to be that teachers should have as much notice as possible about those observations and that principals should have a lot of process that they should have to go through both before and after.”

Other questions are less cut and dry. One issue, Weiner told the teachers last week, is that the state requires districts that use the Danielson Rubric for observations, as New York City is planning, must assess teachers on all 22 of the rubric’s “competencies” — or behaviors and skill sets they are supposed to demonstrate.

“That seems like a lot, especially like in year one,” he said, noting that the city’s pilot Danielson program asked teachers to focus on just six important competencies. He suggested that the department and union are discussing phasing the competencies in over time. “Should we start with three and then six and then nine? Is that too slow? Should we start with 10 and then 20? And we’re really actually struggling with this.”

Soliciting feedback last week, Weiner got a wide range of responses from members of Educators 4 Excellence, even though the group is a proponent of more complex evaluation systems. A 15-year veteran said teachers at different points in their careers should be assessed on different skill sets. Another teacher suggested that teachers be allowed to document some competencies outside of observations. A third teacher said she has found it useful to focus on two or three goals at a time. One teacher said her school rendered an observation rubric “meaningless” by focusing on all of its competencies, but yet another educator said looking at all 22 competencies enables her school, a Promise Academy Charter School, to plan effective training sessions for the entire staff.

Another issue under discussion is one that seemed to have been settled months ago: The process for appealing low ratings. The appeals process sunk a smaller-scale evaluations deal a year ago, so when Gov. Andrew Cuomo launched a full-court offensive to get districts to adopt new evaluation systems, he got the city and union to strike a proactive agreement on the issue. Now, an appeals process for New York City teachers only is written into state law, to go into effect when an evaluation system is agreed upon.

But even within that process, which allows the union to appeal up to 13 percent of “ineffective” ratings, there remain some undecided issues. When the appeals hearings will take place and how the department and union will jointly select an impartial third hearing officer are still up for discussion, city officials told reporters.

And officials are signaling that even if an evaluation system is put into place, some elements might still be up in the air.

For 82 percent of city teachers who do teach a tested grade or subject, the city must decide how to assess their student growth over time. Weiner said the city had piloted 12 different options and teachers had not liked using computerized testing or the assessments the city already makes available to them. But he said other options, including compiling portfolios to show students’ growth and setting individual goals for each student, were more popular.

“We are probably going to allow some flexibility in this area for schools and teachers to work on this,” he said.