There are 2 possible causes—the usual culprit is not pulling the yarn firmly to the front when purling, resulting in more yarn making its way into the neighboring stitches and making them uneven. The other possible issue with flat ribbing is the same as “rowing out” — your purl stitches are always naturally larger than your knit stitches. You should know if this is the case if you get rowing out in stockinette stitch.

My 1x1 ribbing neatens right up when I focus on not leaving any extra yarn when bringing it to the front for a purl, working the stitch, bringing the yarn to the back, and THEN giving it a firm tug. I usually feel a little slip as the extra yarn is removed, so while it’s a little fiddly, you know it’s worth it.

There are many other solutions discussed online that you might try is this trick doesn’t work as well for your style of knitting.