My bet would be that you missed the back of the stitch. When you do a m1 and have to knit into the back, it's very awkward. It's a tight little loop that you have to really finagle your needle to get into.
What I do is to slip my needle into the front, as if I'm going to purl, and then, keeping my needle in the stitch, kind of lift my right needle over the left while both needles are in the loop. Then it ends up in the right spot.