I have that problem all the time with k2tog (and k3tog, for that matter)...try this trick: slip the first (right) stitch purlwise to your right needle. Then take your right needle and put it in the second (left) stitch and wiggle it around a bit. Then slip the first (right) stitch back to the left needle, and then try your k2tog. This has really helped me. It doesn't always work on the first try, but I find that if I can get that left stitch a bit looser, the k2tog goes easier.