I needed to re-design the motor mounts on the robot base, they didn’t hold the motors securely enough and tended to shift around.
The new mounts are better fitting and include a small wedge that fits between one of the gearbox plates where there are no cogs.
One of the great things about 3d printing is being able to pull out part of a design and iterate over in isolation, then when you’re done incorporate it back into your main design.