I agree. The motor/wheel combination is a student favorite and the soft plastic does not cope with even slightly inaccurate insertions of the D shafts. The plastic is carved and deformed by the shaft and ... just drops off after only 5 or 6 assemblies.
Try to buy spare wheels and you find none in the stores. Littlebits support are helpful and will hopefully make some available but really they should be listed in the shop. You can buy the tethered motors but not the wheels.
Has anyone come up with a better wheel?