I had one get warped in a move now I'm super picky and make them put it in a picture box and stuff around it. I tried keeping it between my mattresses for 6 months still couldn't get it to fix. Try and salvage any that does lay flat and have a small board and go get a Joann coupon and get a new one! Didn't know they were made of cotton cause their price sure when up to...Hahaha

Mine flattened out perfectly after leaving it outside on patio tiles in the hot NM sun for a day.

I have fixed mine several times. I put it on top of my clothes dryer and stack some heavy books or whatever on it.
Then I dry a big load of towels. The top of the dryer gets nice and warm that way. I leave the cutting mat there with the heavy stuff on top of it til it is completely. Voila!! The mat is not wavy anymore.

Chris

Wow - love the hot bath with vinegar suggestion - will have the bookmark this page to later reference - hope you can get the warp out of your mat.