Unable to play two samples at once

I just got my MPC 2500 today, used from eBay. I owned an MPC 2000 about ten years ago, so I'm not a total n00b. I purchased and read the MPC 2500 Tutorial while I waited for this one to arrive.

My problem: I sampled and chopped a drum loop; everything was going fine. But when I try to play two samples on top of each other, it doesn't work. Playing it live, the new sample drowns out the old one (even if I have FULL LEVEL off and I barely touch the pad), and when I do "overdub," the the new sample sounds (and drowns out the old one) until I hit STOP.. And at that point, when I play back the sequence, the new sample is not there, but the old one still is.

I know you can set samples to a "Mute Group" but I haven't done this. I did not read anything about being unable to pay two samples from a chop on top of each other in the tutorial. I'm thinking this might be a(n admittedly strange) defect with my MPC. I only have a short time to return it, which is why I'm coming here with the question. Thanks in advance.

Edit: I probably should have checked this before posting, but I did confirm that it's not just samples from the chop, but any sample. And I tried it on different tracks too. Whenever I play ANY sample, it silences the rest of the sequence.

Thanks a million! I can't believe "mono" is set as the default.

Thanks, man. I tried searching the forums before asking my question, but I couldn't find anything.. I imagine people pose the question in very different ways.

Well, my next problem is that it didn't come with a CF card. I went to Best Buy, and the smallest one they had was 4 GB. The tutorial says it can only use up to 2 GB cards!

I didn't actually buy the card, cuz I read it wouldn't work.. But now, searching this forum and Google, it seems like a lot of people are using 4 GB and larger cards. I just called my wife and told her to bring me one home. Worse case scenario, I'm out a few bucks after selling it on eBay. I gotta test the thing within 7 days and I don't know if I can get a card from online before that!

Tried them all and the smallest size they had was 4 GB. Trying that out right now.

The 4GB card worked just fine. Once again, thanks to Lamdog for helping me out with my problem.. Saved me a TON of time!