DOES the yarn snap back, or is it stretching out and then staying stretched out?
Another thing you might try, so we can help diagnose what's happening: cast-on some stitches, maybe about 30-35 sts, using some other kind of yarn, preferably in roughly the same weight yarn. Knit a few rows, maybe three or four of them. Then just drop the first yarn and start knitting with your corn yarn. What happens?
P.S. Try knitting with the yarn pulled tighter and see if that helps. If it doesn't help, then don't, but if it does, then try casting on tighter and knitting pulling it tighter.