Mason Dixon KAL

sounds like you sewing to tight...if there is sewing involved. i haven't looked at the pattern. but i attempted to make a rag rug once and it looked like a bowl and my mom said i sewed it to tight. hoped this helps
 

I'm making this right now and although I only have the center piece and two strips done, I can see you have to be careful to pick up the right number of stitches (17 I think, or one per ridge) and don't knit them tightly. I used a big crochet hook to pick up my stitches and put them on the needle. so far so good, but that's only 3 pieces laying flat, who knows what will happen as I add more, lol.
 

Yeah, i'd say make sure you are picking up one stitch for every ridge, and for every stitch along the bound off edges. But also, you have got to be sure to bind off very loosely, otherwise the egde in very inelastic. Good luck.