PT10 recognizes MIDI controller on some songs/sessions, but not on others

[joevent]

Hello.

I'm using ProTools 10.3.10 - MacMini (2011) OSX 10.8.5.

I have an old Novation Remote 37SL controller (connected to computer, via USB). I used to be able to use Novation easily on instrument tracks for a specific song (say, song "A"). Then, it stopped working on that song. I'll open other sessions/songs, and it works fine..but, no longer on song "A." By all appearances, all settings/operations are the same.

I truly just wanted to cut one more track, before mixing (of course). Any ideas? (Midi Thru is on, fyi).