I know that I'm late to the party here, but for future reference...
Another possibility is that the loading carriage is out of alignment with the unit as a whole. If the loading cage was installed incorrectly (not aligned with the gears properly), the behavior you are describing can happen. Without seeing the actual machine, it is difficult to be sure, but there are a number of other possibilities as well. It could be as simple as a belt that is slipping once it gets some resistance.