Are you pulling the loop tighter so that it hugs the stitches it goes around? The only other thing I can think of was already mentioned, that maybe you aren't getting your needle through the pulled up loop/stitch and the next one.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper