Tracks disappearing in piano roll

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OK, I'm having a serious problem now...I have a partially finished song I'm working on--when I'm in the Song Editor, everything I've done shows up in the track grid (or whatever exactly that's called). When I go into the piano roll for my organ, orion strings, and synth bass, everything is still there. But when I go into it for my kick drum, snare, and hand-claps, it's empty! What's going on? I can't do any more with the drums now and can't finish the song... :cry:
Question: did you move, consolidate or otherwise relocate the percussion (.ogg, .wav, .sf2) files around on your hard drive? If so, LMMS can't find 'em. Learned this by bitter experience.