How to properly enforce consecutive time slot constraints in a constraint programming timetable software?

Question

Problem Description

I am trying to add a constraint to my scheduling optimization problem to ensure that generated timetables have consecutive hours (i.e., no gaps between time slots within a day). However, the current implementation does not work as expected, and it returns INFEASIBLE despite having sufficient resources (teachers, classrooms, and subjects). Below is the problematic section of my code:

# C - Problematic - Timetable needs to be consecutive
for lvl_idx, level in enumerate(self._problem.levels):
    for sub_idx, subject in enumerate(self._problem.subjects):
        if self._problem.curriculum_contains(level, subject):
            required_slots = self._problem.curriculum_time_request(level, subject)
            
            for day, (start, end) in self._problem.slots_per_day().items():
                for tch_idx in self._all_teachers:
                    for loc_idx in self._all_locations:
                        for slt_start in range(start, end - required_slots + 1):
                            sequence = [
                                self._assignment[lvl_idx, sub_idx, tch_idx, slt_start + offset, loc_idx]
                                for offset in range(required_slots)
                            ]
                            
                            for i in range(len(sequence) - 1):
                                self._model.Add(sequence[i] == sequence[i + 1])
    

What I've Tried

• Added more teachers, removed some levels (class schedules rather than rooms), with no success. The variables don’t seem to be the issue.
• Changed the position of this constraint in the code to see if the order affects the results.
• Removed other constraints to isolate the problem.

Context

I am building software using OR-Tools to generate timetables for high schools in Romania. I’m working with a slightly modified version of the OR-Tools school scheduling SAT example: OR-Tools school scheduling.

Here’s some background information that might be relevant:

• In Romania, students are assigned to specific classes (rooms), and teachers move between these classes.There are exceptions, for instance, in IT&C (Information Technology and Communication) subjects, we need to go to rooms with specific equipment like computer labs.
• The breaks between lessons are minimal, typically around 10 minutes. (no gaps between hours required, at all!)

Here you'll find my full code to replicate the issue, along with all relevant variables.

Thank you.