We are using 4 servo motors to drive our robot. We have to go up an incline of 30 degrees. When going up the incline one side(either one) will stop working. Need help figuring out what maybe happening.

Sounds like a rated power issue... that is the motors are not rated for the
Load you are applying to them. See if you can prove this by either removing weight or reducing the incline.
Then consider higher power motors, maybe increasing one on each side would work...