Printer problems

I am trying to print a double sided postcard on 110 lb cardstock. I printed the front side just fine with no problems. Now I'm trying to print the back, and the printer jams everytime, and I can't get it to print at all. I think the paper might be warped a little from going through the first time, so I tried to leave it al night pressed between two big books, and that didn't help any. Is there anything I can do to fix this?