The self propelled mechanism on these is rather troublesome as I have found in the past, however to stop the rear wheel locking up while being pulled backwards there is a quick fix that will make it push smoother at the same time.
Underneatht each back wheel (remove hubcap and clip holding wheel with a flathead screwdriver (watch for where the clip goes, because sometimes they take off at a rate of knots...) there is a small gear that should be removeable underneath the wheel, once this is removed put the back wheels on, the clip may need some encouragement to put back on, and it will roll a lot easier.
