Saddle problems...please help?

i have two horses and am having a heck of a time trying to find saddles for them. they are both quarter horses, both 15.2 hands high. i measured each of them with a tape measure just to get an idea of how similar they are and they both have roughly the same measurements around the girth and length of their back...the only difference is my gelding has higher withers. first of all, there are no tack stores anywhere around here and that also means no professional to come help me fit them. the previous owners of the house i bought left one saddle behind. i put the saddle on without being cinched and without a saddle pad to just see how it fit to the horses' backs. i placed the saddle 2 inches behind the shoulder blade on both horses. i stuck my hand under the saddle and ran my hand down the length of the saddle tree to see how it was fitting against their backs and it was putting a lot of pressure on my hand by their shoulders but as i ran my hand towards the back of the saddle, the saddle gradually got looser and looser to the point where it's not even touching either of the horse's sides at the back of the saddle. its just lifting up in the air without even being tightened down. does this mean the saddle tree is broken? or does this mean the saddle tree is too narrow? it's a western trail saddle and i have no idea what size gullet or saddle tree it has either.