[Manoel]

The CIMs are, in theory, pretty symmetrical (no huge angle difference as in the drills), so that should not be your major issue. You even made both drills turn the same way, so your motors are not likely to be the problem. Look for gears misalignment and, above of all, different chain tensioning from side to side.
Other factors might be shaft bending under load (robot OK on the table, bad on the floor) and, less likely, uneven weight distribution between the left and right sides.
Do you identify any of these in your design?