Testing was sucessfull, but the generation fails

1. Try to generate on higher complexity

2. Try to generate with relaxed constraints to see what kind of constraints had to be relaxed, and then relax them.

3. You can also try to test bigger parts of the timetable. The Test tests just one item at time, however sometimes problems are not linked just to single class/teacher, but occur when trying to generate two or more classes/teachers, whole grades, or even half of all classes, etc...
Please see this page:
Testing multiple classes/teachers at once