If you need to calibrate your PID settings, use g code M303 to run the PID autotune. You can plug the numbers it gives into your firmware as a good starting point. Actually, I never changed any of the values it gave me - I just entered them and i was happy with the rexults.
If you feel your PID is properly set, check the voltage at the board connection points. You should see 12v if the mosfet is switched on full - so do it when the bed is cold but is heating up.
If you see 12v, the problem is most likely your wire. What gauge are you using. The bed needs pretty heavy wiring. I would say AWG 20 at least, 18 if you have it.
If you don't see 12v then the problem is most likely on the ramps itself.
