I agree with Kitten Val.
I don't have that exact pad, but I use the Poly Pads Classic pads, they are similar to the style that you have with no girth or billet straps. I also use quilted cotton pads with a Mattes sheepskin 1/2 pad on top that doesn't have straps and I have never had any of these pads slip.
As Kitten Val mentioned, check the saddle fit. If your saddle fits well, when you are tacking up, make sure to pull the withers area and spine of the saddle pad up into the gullet of the saddle and that your saddle is sitting in the correct position on your horse's back, then make sure your girth is done up tight enough to hold everything in place. I think the cause may be as simple as not having the girth done up tight enough.