Western saddle pops up in back

I bought a new western saddle today, and when I tried it on Flash, the back of the saddle pops up by about an inch when its cinched tight. All the measurements are correct, and it seems to fit in the shoulder area, and also has back clearance when someone is sitting in the saddle. Is it normal for new western saddles to do this until they're broken in?
My boy's the type of horse who will not move or listen to cues with a poor fitting saddle, and I rode him around at a walk, and also lunged him at a trot and canter. He moved great and listened to cues, so I know its not bugging him. The sweat distribution was also even. Once someone is sitting in the saddle, the back goes down like it should. You can also fit your hand in between the saddle and shoulder area when someone's sitting on him, so I know its not pinching. Does anyone know why it is doing this?
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How is it rigged?

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Posting pics of the saddle on the horse, will help gain more feedback. Because without it its hard to know which issues it could be stemming from.

That's what I was going to ask. It may be that it needs a rear cinch.

A rear cinch is not for correcting an ill fit. It is used for keeping a saddle down when going down a steep hill or when roping a steer.

Does your saddle fit properly without a pad? Can you show us some pictures?

I will try to get some pictures tomorrow. I had it done up with a regular front cinch.
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I am new to western saddles but I have been told that if the saddle is designed to have a back cinch it should be used. I recently bought a little western saddle for my granddaughters little pony and it fitted well, except it lifted at the back at the trot until I used the back cinch and it was fine! I love my western saddle! It's a Clinton Anderson designed one and fits my Arab very well!

A rear cinch is there for roping, not for holding the saddle down. If the saddle is full rigged, with the cinch all the way forward, it will sometimes lift some in the rear without weight on it. My Circle Y is set up so I can choose full rigged, 7/8 or 3/4 rigging. With 3/4 rigging, it doesn't lift in the back.

It can also be caused by a saddle that doesn't fit, and then the solution is a saddle that DOES fit.



Western Saddle Rigging

It fits the same with the saddle pad off as well. Looks comfy in the shoulders, but lifts slightly in the back. I know if your tree's too wide it can do that, but the problem is that this tree seems to fit the shoulder area fine. I've tried tons of saddles on him, and this one seems to fit the best. If I went with a smaller tree, it would pinch his shoulders. Then I thought that maybe the back part was too curved, but when your sitting in it, he has plenty of back clearance- not too much mind you, but enough that its not touching his spine anywhere. I only found one saddle that seemed to fit him well and didn't lift up in the back, it was 40 years old but unfortunately its my BO's favorite saddle and she would never part with it. We took measurements off that saddle, and this one has those exact same measurements and fits exactly the same in the shoulder area. I'm stumped.
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