Refuses To Jump
If the saddle doesn't fit, it will probably be uncomfortable to your horse. The surface of the underside of the saddle should follow the contours of the horse's back so the weight of the rider is spread evenly over its surface. If it doesn't fit, this means that certain areas will bear more weight and pressure points will develop. A horse will sometimes cope with these pressure points under flat work, but when the horse jumps, or rather, when the horse lands, the weight of the rider will be magnified considerable when their downward force pushes onto the saddle. This will cause the horse to feel pain or discomfort when it lands. It will remember this pain and be reluctant to jump in the future.
Use The Perfect Fit Saddle Fitting Kit to check if the saddle fits. If it doesn't, be sure you get one that does and don't ride him in the old saddle until you have. Using the kit, check the saddles's fit at least every month, or more often if there are changes to your horse's diet or exercise.
When you are certain that the saddle fits well, it could be that your horse remembers the pain that he was in previously, or, he is still in pain from the damage done by that saddle. If this is the case, you must have his back checked by an equine physiotherapist, chiropractor or osteopath. In fact, if a horse exhibits any unpleasant behaviour, even if you don't suspect the saddle, the first thing to do is have him checked out by one of these professionals.
Sometimes it can take a while for a horse to trust that the saddle isn't hurting him, so be patient with him if he takes a while to accept that there is no pain – this process can take many months of patient handling.