The DM42's USB connector is close to the tolerance regarding its distance to the edge of the calculator. If a cable is used of which the length of the actual connector is also close to the tolerance then the casing around it might well come into contact with the DM42 if that casing is a bit bulky. This could prevent you from inserting the connector far enough. We're only talking about tenths of a millimetre here but that's clearly enough to get in the way.
In order to test this theory, try removing the rear case and connecting the USB cable again. If you now have a stable connection then connector length/recess is indeed the problem.
In this case, one solution is to shave some of the plastic off the top part of the connector housing on your cable. Observe the photos below:
BEFORE:

AFTER:
