1) Built up junk on door tracks at locked position.
2) Linkage from operator to car door needs looked at for loose bolts on driving arms.
3) Shorting SE or PHE cable
4) Broken limit collar...or maladjusted limits.
5) Door belt deformed on the motor sheave at full closed position.
6) Door belt walking off of big sheave near closed position.

All common stuff, but #1 is common on Dover.

Hope you have a FAST tool if it’s not a mechanical issue.