This is most likely due to the RAM on the MPC being filled up and not having enough left over to sample with. When the MPC starts up it can load samples from your project into RAM. Check your available RAM to see how much space you have left to sample with.
To solve this problem, make sure that you are editing your samples down to only contain the few secods that you need for each sample. i.e. if you sample 20 seconds and use only 5, edit out and delete the extra 15 seconds to free up space.
Refer to your manual on how to edit samples.