Garage Band Jam Pack Orchestra Problem. Sounds SOMETIMES won't play.

This keeps happening with some sessions and not others.. I'll do a few tracks on a session---drawing on the jam pack instruments. Then I set up another new track. None of the new jam pack insturments will sound, though, when I play the keyboard, the midi is being put down (it can play back on the piano or original garage band sounds). No adjustsment made to tracks/volume, etc.

If I open a new session, than, most times, the new instrument will sound.

When it's not working, I still, again, see it light up on the LCD and record the midi.

What I really don't get is why it happens sometimes and not others. And it's not just about tons of tracks---happened the other day just the third track I added.

I do have an old keyboayrd with a midi adapter, but, again, it works fine most times.

Thanks.