Everything Delta_G said +1.
Also, you mentioned that you implemented a code for the servos to go from 0 to 180 and back. We call this action a sweep as can be found in the sweep tutorial.
You mention that at least one servo would move to 180 but not back.
Do all you servos behave this way? Or only some?
If different servos behave differently, I propose that you have broken servos.
If all the servos misbehave, I suggest that perhaps the code is at fault.
Can you post the code that sweeps your servos back and forth? Use code tags.