Doesn't work well
Some horses have particular problems when they are working. Some will have difficulty striking off into canter on one leading leg, some will just have difficulty going into canter. Then there are those horses that are stiff on one rein and unable to flex in that direction. Some horses can't go straight. Some horses are unable to show much lengthened stride. Some horses can't go on the bit or just dont move well at all.
Of course, there are many possible reasons why horses and ponies have these problems, but a very important factor could be that they are restricted by the poor fit of their saddle.
Many people will say that they have had the saddle fitter out to fit their saddle and because of this they feel that they have done all that needs to be done. But, the saddle fitter can only be sure that the saddle fitted on the day that they fitted it, because both the saddle and the horse can change shape. A fortnight later both could have changed shape, particularly if the saddle is newly stuffed and/or the horse has had a change in diet or exercise.
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 inability to work well, 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 become relaxed. Sometimes the upset that the pain causes him can create an unbalance of the healthy gut flora. This, in itself, is enough to cause your horse to be tense and exhibit undesirable behaviour. This unbalance can last a long time, so it is always worth giving a supplement of probiotics to a difficult horse.