My saddle appears to fit my new mare fairly well, she has plenty of clearance, doesn't rock back and forth, and isn't pinching her shoulders. The problem is that the saddle tends to slip to the right or left. Is this a saddle fitting issue? Or is it just because my mare is a 2 x 4 right now? Is there any way to stabilize it? And yes, I have made sure that the girth is tight.