Spa cover not sealing in cold temperatures.

I purchased the CVR-RSQ-RHT Spa Cover at the end of 2009. I noticed last night the cover was raised on the two ends. I used the clips, which were tight when I connected them last night. I had to lean on the cover to get the clip connected. I noticed this morning the ends were approximately 1/2" up off the tub. The temperature overnight went down to 0 deg. I'm thinking the foam or vinyl contracted in the cold and lifted. I will probably resort to using rope to tie the cover down. I did check for obstructions such as ice under the cover. I use the tub about 3 times a week. Our temperatures have averaged 25 deg at night until this weekend. We have had snow almost weekly since Christmas.

Do you have the order number for this spa cover so we can review the cover selected to determine what is causing this?