If Sue's suggestion isn't it then I suspect you're not knitting on the "purl side" of the garter stitch. Look closely at front and back side of the stitches up close to your needle. One side will have a "purl bump" and the other won't. When you knit garter you are knitting on the purl bump side. You might have to rip back one more row to get there.
Here's a crop of some garter to show you what I mean.