I think whether a mat slips or not may be due to the carpet it's sitting on. We never had a problem with mats until we moved to this house, which had just been re-carpeted throughout with very high-quality synthetic carpet. In every room, any rugs we put down have walked. Sticky backing, rug grippers, double-sided tape invented for the purpose - nothing has worked. We even bought a VERY heavy coffee table (takes us both to move it, in two separate pieces) but guess what? The rug underneath the table still walks, while the table stays where it is! Short of re-carpeting every room in the house, we can't cure the problem. So every day we go round pulling the mats straight. Sigh...