This may be one of a couple of things:
Make sure that your students are exiting the lessons correctly. They may be getting
to the end of the lesson and pressing the 'Restart' lesson button instead of the
'Exit' button to go back to the lesson manager.
Make sure that your students are completing the entire lesson. If they are using
'Reading Horizons, they may have to complete certain sections of the MDP before
moving on to the next lesson. (This is a setting in the Administration system.)
If they are using Discover Intensive Phonics, they may have to complete the practice
and story sections at the end of the lesson before they can move on.
Make sure that the student has not gone back to review a lesson and left it unfinished.
If any reviewed lessons have been marked as "failed" or "incomplete," the student
must finish them before moving on.
It is possible that this could be a permissions issue; see the permissions instructions