Without more info/pics, can't do more than guess, but it is common for horses to be stiff/stuck through the shoulders & hips, that makes flexibility out to the sides very difficult. Saddle issues are a common cause of shoulder pain/tightness. Often it's only noticed during farriery... when the farrier comments he has to work in a less comfortable position, but when adjustments have been made, horse is loosened up, they often move quite differently, showing that they feel better... showing that it wasn't just farriery when THEY noticed they had a problem.
**Over here at least(& guessing probably same in US), 'horse chiropractics' is not a regulated industry(!
), just like farriery, so beware the 'expert' you choose - they may have only done a weekend course in bone cracking, for all you know! A registered veterinary chiropractor however, must be a vet or qualified human chiro first, before doing a *proper* chiropractic course, so you can be sure there is a good level of knowledge & experience there at least.