Heel slippage arising from a dispensed orthotic is not an uncommon problem. Ways to address this problem include modified shoe lacing (lock-lacing) and tongue pads. For a how-to on lock-lacing, see my prior post at http://tinyurl.com/cu3r4ml  .
To make a tongue pad, cut 1/8-inch adhesive felt in an oval shape and apply it to the underside of the vamp of the shoe, just distal to the throatline (see photo at left). Apply this to one shoe first and have the patient compare it to the shoe without a tongue pad while walking down the hall.
This method works for both slip-on and lace shoe styles. Your patients will be delighted by the effectiveness of a tongue pad and appreciate your ingenuity in solving an otherwise annoying orthotic problem.