knees slip out of ab circle?

What can I put on the part where i keep my knees, so that i won't slip. i actually love it, but the downer is is that i start feeling that i'm going to slip from it after 30 seconds and sometimes do slip from it.I looked at reviews and I know that a lot of people have had the same problem. I didn't buy this new, I bought this for 30 bucks at a thrift store. I think that they may had sprayed something on the knee pads because i didn't start slipping from it till after a few times using it.