Uneven stitches

Looking at a piece of work done in stockinette stitch from the front, I see that my stitches are uneven - the left side of the stitch is much tighter than the right. What causes this, and how can I correct it?

I remember reading an article that explained how the twist of a yarn can affect the shape of the stitch, e.g., if your stockinette stitch comes out looking more like a series of little checks rather than symmetical V’s, it is probably just the nature of the yarn and not you. I can’t remember where I saw the article, but maybe someone here has a link for you. I think that there may have also been a previous post on this topic. Anyway, you might try knitting with a yarn that has a different twist and see if it you get the same result.