Crest Ridge Saddles has lots of information on checking saddle fit for a western saddle and how to make a back map. Checking Saddle Fit
How does your horse do bareback?
Have you ever tried warming the horse up and stretching him/her out a little before you ride? I have a horse that has a cold back and I can tell if she is tight, especially if her tail is crooked. If I warm her up and stretch her, she does much better. She is only 14 and has beautiful conformation but her back troubles her. If her back is hurting, she is cinchy and grouchy. If I stretch her and warm her, she is a princess!
If it is not an issue of pain, you may have a horse who is barn/buddy/work sour. I have a sweet Appy gelding that tried balking a few times with me. I worked hard to start my cues as light as possible and got more firm as needed, with the "squeeze, cluck, spank," but he still wouldn't go. When I spanked him on his butt, he would buck, and it seemed to make him more sour. So I switched to spanking him behind my calf as I had read somewhere. Still wouldn't move, although he didn't buck.
I did not feel good just spanking and spanking, so I got down off his back and lunged him, changing directions frequently and working until he was more responsive. He was a stinker at first, staring stubbornly at me and kicking out in the air. But he had no choice but to move on the ground, and I was safely out of his reach.
I lunged him for a minute or two until he was moving quickly off my directions, and licking and chewing. Then I got back on him and cued him forward. The first couple times we went through this, I had to get off and lunge him 3 times during my ride. Now, he is fine. He will try it on someone else if he thinks they will fall for his tricks.
Horses can be complicated critters...