First thought is that You did not center servo properly or set it all the way to the right before gluing the pot. Second thought is (not knowing how well You understand the code You work with) that PWM is not set correctly.
If You have spare servo that has not been modded, connect it to Your microcontroller and check if it behaves as expected. If it does, it means that code is fine and that problem is with the modded servo itself.