When it's not the saddle - it could be the girth... A big shoulder or a girth groove at/in front of the saddle, can make a saddle that fits well, move up on to the back of the shoulder. Causing the horse to lose movement, and be on the forehand. These shaped girths can help with the misalignment.
Dr. Deb - Lessons from Woody PDF Reading 1, 2 and 3.'Woody' helps explain the impacts of a horse not being straight. Basically, on everything!
Saddle Pads with pockets for 'shims': Uberhorse Supply the stunning Mattes Correction Pads. True quality sheepskin half pads, saddle blankets with pockets for shims/inserts to tailor saddle fit. Available in any colour combination - limited only by the limit on your credit card!!
Carousel Equestrian Suppling high quality Lambskin products that wont break the bank - Half Pads or saddle blankets with pockets (to adjust the fit of your saddle).