If we can see a picture of your saddle on the horse it would help. What kind of saddle and pad are you using?
Deb Sibley Ultra Flex with a 1/2in wool fleece pad and a shim because he has atrophy behind the blades from other saddle. Thinking about using the CE Biofit until he fills in but not sure how the sibley will work with it. I have two Sibley's and a billy cook. I was using the Billy Cook on him but I obviously didn't pay close enough attention and it didn't fit. I use a Tex Tan to rope with and it seems to fit just fine, even sweat marks, no soreness (he hasn't had any with the sibley either just the funny sweat marks) and moves out and pulls good. This is the same sweat pattern I get with the Sibley every time I use it on him. This day I worked him hard and figured it would show the most.