In order for your CPM therapy to be effective, your limb must be properly positioned in the CPM machine.
If your CPM Machine is for your knee:
Make sure your knee is lined up with the unit's hinge and your foot is rested firmly, yet comfortably, against the footplate. Use your machine on a flat, firm surface, such as the floor or your bed. You may prop your back up for comfort, as long as it doesn't exceed a 30 degree angle.