Autosampling

I've been autosampling on my MPC keys 61 for some time now with no problems. The sources have been a variety of hardware synths, vst synths from Logic, samples from the Logic sampler etc. But the last few days, I sometimes can autosample - and sometimes not - and I really can't figure out, what I am doing wrong. Just the last hour I couldn't autosample (no sound coming through - but I can easily get sound from my Mac to the MPC - it just doesn't respond to MIDI. Then suddenly - I could autosample - and now 15 minutes later - I can't sample again. It doesn't seem to be the MIDI to USB-cable. And - it's the same setup as always (midi out from the MPC to USB and into the Mac. From headphones out on the mac a minijack cable to 1/4" jackcables put into th Input 1 and 2 on the MPC 61 Keys.
Now I'm ready to call an excorcist or something like that - but first - does this sound familar to any of you? And - does anyone have an idea for a solution?
Best regards,
Stefan, Denmark