Is the saddle sliding back during your ride? Or the cinch sliding forward? If so, you may not be positioning the saddle correctly, or your horse's girth groove may be too far forward for your saddle's rigging.
Try riding the saddle with a 1/2" wool felt pad. If the saddle stays put better with a thinner pad, then the saddle may not fit, or may fit so well that you don't need a thick pad
Pictures of the saddle with pad before and after your ride, and with no pad, would help.