Some of the switches under the MPC's buttons will eventually need replacement. This can be indicated by the fact that you may have to press harder to get the button to respond. It is very important to get your MPC serviced ASAP from this condition. Most often many put off this needed task and will also end up breaking the plastic button over the switch.
The switches themselves are still available from Akai and cost just a few dollars each. I recommend taking it you your local service center for this repair. As the repair itself is a relatively simple, the MPC needs to nearly completely disassembled to do this. The MPC is very expensive and there is a great risk of damaging components or yourself if you attempt to repair this yourself.