You can check a staff member's availability by clicking Appointments > Boards > selecting a board.

Under the staff tab, click their name.

Next to their current schedule, click the dropdown menu and Edit.

Update the default schedule and save.

Then consider adding / editing a Service Override. This controls when the instructor is available to perform a specific service (overriding the instructors default availability).

If this doesn't resolve the issue, check the board to see that the correct venues are assigned, that they each have the correct schedule set, and that the correct services are assigned assigned to the board.