You're collecting those "knowns" with the VB-MAPP App and you may not be using them (gasp). Those known targets that the students have shown to be mastered need to be used (exercised).
The one obvious value of collecting knowns is to properly document what the student has demonstrated mastery for, but we should use them for more than that. While providing instruction so that the student learns even more mands and tacts, it is often helpful to intersperse easy and hard targets where the hard targets are the new items that the student is learning and the easy targets being the items the student already "knows".
So use those knowns as the easy targets during instruction to keep the session lively and reinforcing for the student. When you sense that the student is getting frustrated with the new material, throw in a few "knowns" in a row to get the students spirit up and ready for a couple of the new targets.