Won't play old midi notes with Kontakt

I have a very weird problem that some of you might know the answer to. When I open old projects, as in before I bought Native (and created in Artist, not Pro) , everything sounds good. I sometimes would like to exchange instruments for the ones from Kontakt, but then the sound dissapears. I can still hear the sound like from the midi input, but the old recorded notes won't play. It's only Kontakt. If I choose a synth like FM8, sound is back. Also if I rerecord the music, Sonar plays the new notes via Kontakt but not the old ones. Any ideas ?