Likely it's a reassignment to a different midi channel like Ramtitam says. Right click the "I A-E" text at the top of the mix window, select the instrument view, and check what channel the midi is assigned to. This happens to me all the time (apparently an update has fixed it). Occasionally an instrument track reassigns itself when inactive, and starts sending it's midi to a completely different instrument track while I'm working on it! That can be a real pain, because I have to look through every instrument track to check if it's the culprit. Good luck
