Playback Sound Stops Working Unexpectedly

Without uploading the score and giving steps to reproduce the problem it would be difficult to reproduce. I think it would be easiest to troubleshoot if you uploaded a score where you saw the problem and indicate some of the notes where you remember there not being any sound. I have some suspicions, but I'd rather verify them.

I'll go out on a limb with one guess, though you might be entering notes that are too high or too low for the instrument in question and thus there is no sample for them in the soundfont you are using. But indeed, attaching the score and steps to reproduce the problem would help us say for sure.