How to keep couch cushion from sliding?

After a year of use our couch cushion keeps sliding off faster amd faster when we lay against the pillow. It presses in the crack. Anybody got an idea on how to prevent it? 1st picture you can see the pillow pressed in. 2nd is the resulting gap. 3rd is the surface of the couch and the cushion. We tried a rubber mat we also use for some small rugs, but it doesn't work at all. Do you think some Velcro mats could help or would they also be too weak?