[Brian C]

One other item to add. Sorry if I missed it being posted earlier.

Electric motors are designed to be more efficient when operating in a certain direction. The armatures are timed with the magnetic field via the commutator brushes to give more torque/power in the desired rotational direction of operation. When that same motor is operated in reverse it will have less power. Depending on the motor maybe not significantly less but definitely less

When your robot is going forward one set of motors is spinning in their "forward" direction and the other side is going "backwards". When you go in the opposite direction the other side will be less powerful.

This is a common reason to see the issue you're having. Some teams have done a very simple fix to this in software by only allowing one set of motors to go to 85% throttle in one direction and the other set to do the same when in reverse. While not overly scientific it usually does the trick.