Why can't I put a lesson where I want it to go?

There will be a very good reason for this.

To see what the reason is, just click on any period in the 'dot row' (.) on the Class Timetable Strip or on the Staff Timetable Strip - here is an example:

These strips are shown on the Visual Builder Screen (or by double-clicking on the Priority Screen) - see sections E12, E13 in the Manual.

The Report Screen will then pop-up and tell you exactly why you cannot place the lesson in this position:

See also:
- Solving 'Kickouts' 
- Solving problems in the sixth-form