Help with MIDI input

Greetings,

I have been using a MIDI keyboard for note input for a while. But now I am experimenting with some orchestral pieces and I'm struggling with some inconsistencies between the instruments.

Basically, I start the score with only the bass. MIDI works. I add viola, MIDI doesn't work on the viola but still works on the bass. I add violin, now MIDI doesn't work on violin but works on viola and bass. I add another violin, now it works on the previous 3 instruments but not on the last violin. And finally I added a cello and now it doesn't work on the last 2 instruments I added, but still works on the previous 3 instruments.

I really can't figure out what I'm doing wrong.

Any advice appreciated.

Thanks...