Make sure the feed dogs are perfectly straight in the throat plate slots. And then make sure that the presser foot is sitting straight on the feed dogs. If they're not, loosen the presser foot bar slightly...& turn it til it's straight. The only other thing it could be is that the presser foot is worn pretty heavily on one side or the other...or that the feed dog teeth are worn much worse on one side than the other. Good news is that it's all fixable!