Emily I'm sorry it's sliding. It can do that on some dogs, we know it did that by the end of the day when Jerry wore it. If a dog has a lot of fluff it can often stay in place but for smoother coated dogs it can slide. Without a front leg, there's nothing to hold it there. As far as we've found though, it's still the best all purpose (not just for walking) one available for front or rear Tripawds. 

The harness has five different adjustment points including two on the chest.  Did you try playing with all of them? If you'd like to post a photo maybe I can give you some pointers.