[BBray_T1296]

If you try moving the wheels by hand, does one side seem more difficult to move than another? Does one side of the drive sound differently to another? If you have more than one motor per side, make sure all motors are actually running (try one motor plugged in at a time and see if you notice anything). Without sensors to read it could be difficult to tell, though sound is usually a decent indicator

Check that all your motor controllers are properly calibrated. Check that you are using the same motor controllers on all motors in the drivetrain.

If you are using chains/belts, check tension, perhaps one is tighter than the other and causing issues. Did you lubricate gears? Is anything potentially bent in a weird way? Is your drivetrain square (that is are both sides parallel)?

Chances are the problem is a mechanical or electrical one. It is not impossible but most likely not a software problem, unless you are trying something fancy coding-wise.