Only move on when you’re ready and confident you can complete the next set at the prescribed amount of reps. For example, if your program calls for sets of 12 on the pull-up bar and you can only perform seven or eight with the next resistance down it may not be time to move on yet.  Program a few extra sets with the stronger band and work through that until you can reach the prescribed. Conversely, if you can crank out 15 reps with a strong band and your program only calls for 8 reps then it’s time to move on.