Instrument track doesn't record!

I have a song with a couple of audio tracks and one Instrument track (with Precense). It all works fine. So far so good.
But when I add a new instrument track, pick an instrument and record it it simply doesn't record! It sounds correct wheter I use QWERTY or my exteal MIDi-KB that I always use. I can even see the "lines" in the track as long as I press a keys,and it follows the notes. But as soon as I release the key it disappears. Nothing gets recorded as if it was "record protected".
I can't even record in the first existing "working" track.
When I create a new song I can create as many instr tracks that I want and it works, but it refuses to record on this specific song (and some others, it has happened before.
Problem I am missing something really basic/fundamental. If so, please tell me what before I get crazy(ier).
Brgs

Pelle