help! !

I am new to my MPC, so I'm not too sure if I am just vague, it may just be a setting but I have checked plenty of times.. the problem is I am trying to make a drum beat and layer my kicks etc, but when I have tried to layer, the first recording is cutting off the kick I want to add in (using overdub), it is strange. I have every setting on POLY, so I don't know what the problem is??
Help! Cheers, Zoe.

do you have the Program set to POLY as well as the individual samples?