No JJOS for the 500 so you good. Wished JJ got on the 500 but its for the 1000 and 2500 exclusively.
Could be a polyphony issue. If you sample in stereo then that reduces the MPC to 16 polyphony. For the full 32 polyphony, all samples must be mono. Of course, you can mix and match stereo and mono samples, just keep in mind that stereo takes up two polyphony and mono takes up one. So for every stereo sample, minus two from 32.
What I do, and its just for thought so do what you do, but this how I do.
I use a Y cable and plug the L and R into the output of the preamp and the 1/4" mono jack into the 500's L input and sample in mono all day. From the preamp, I can tweak the balance and sample from either the right channel or the left channel without pulling out cables. I'll have two pads in mono but I can pan one to the left and pan the other to the right and then its stereo at a cheaper memory price. Plus you can layer samples to a pad so instant one pad stereo coming straight out of mono.