How can I fix my GMC MOS230 Orbital Sander?

When there is no sanding attachment on the machine is works fine, when the round sanding disc is placed on the machine it works fine. When I place the 1/3 square pad on the machine it won't work, the motor still comes on but the pad doesn't vibrate.
The motor sounds to be jammed and not spinning. As soon as you remove it and put the round disc back on it works fine. Is this a problem with the resistance in the motor or the attachment, as there are two other attachments that work on the same principal as the square pad and they won't work either. Cheers.