How do you record one sample over another?

For example, I bang out some pads and create a nice little loop. The next thing I want to do is bang out some more samples over top of the loop I created either on the same track on overdub or on a new track, doesn't matter. However, when I press on the pads, instead of overlapping the original loop I created, it cuts it off. I've already played with the POLY and MONO settings, not sure if this is what I'm suppose to be messing with or not but either way, I want to be able to play over top of the sample without cutting it off, how do I do this?

Change tracks....

each sound set should get it's own track, some of my sequences get roughly 8 - 14 tracks depending, sometimes more or less too.

i.e.:

track 1 - kick & snare
trk 2- hats
trk3 - percussion

etc

etc

What MPC do you have?

If it's a 2000XL, you need to check the pad cutoff settings.

Hit Shift+4, then Drum 1 (f1), then PARAMS (f2).
then Highlight MONO and press open window (the orange button), this will show you if any notes are cutting any off.
This is Page 126 in the 2KXL manual.