Question about Sonar X3 Studio with Session Drummer 3

Hi,
 
I use Sonar X3 Studio with Session Drummer 3. With the B key I open the pane where the MIDI drum patterns are. Normally I can preview those by single clicking on them. With a simple movement of my mouse I can transport a pattern that I like into a drum track in my project.
Sometimes only some of the patterns can be previewed. Other ones in the same folder don't sound, and contents of other folders don't sound at all. If I drag those patterns into a drum track, they also don't sound there (where previously loaded pattern still do sound).
 
Any suggestions what might cause this?
 
Greetings,
 
Rob.