Problem changing previously entered drum set note/Instrument to a different note/instrument

Can anyone help me understand why, when using the drum set score, I cannot always click/choose a particular "note," with its associated drum set element such as hi tom, and change it easily to a different drum set element, such as snare drum. Weirdly, it appears I can change the note/instrument only in one direction-- moving up the staff--, say from a bass drum note to a snare note, but not the reverse. My experience is that trying to change an existing snare note to a bass drum note, moving it down the staff, seems to jump the bass drum note to the beginning of the measure, not putting the new note/drumset element where the snare drum note was originally. This problem also exists trying to replace a rest with a note/drum set element by selecting the rest and then trying to select a note with a rhythm value and corresponding drum set element, such as snare. Again, I am finding this sometimes works and other times I cannot make the replacement of the rest with a note/drum set element. If anyone can help me clear up the confusion this is causing me, I would be very grateful.