How do I get the yarn to knit straight?

I’m knitting in stockinette stitch with big loop mohair. It comes out longer on the right side of the piece than the left. I’ve knit it from both ends of the yarn. Any suggestions?

Longer on which side? You may be adding stitches, see if you have the same number you cast on. Or maybe if you set it down in the middle of a row you pick it up and knit the other way as if you were at the end of a row. So half of it will have more rows than the other. A picture would help.