You're doing it wrong!

[pne]

I thought I was doing the Ian Knot, but it seemed a bit fiddly and I kept using my thumb and index finger to pull the left loop through the right one, even though I remembered the description talking about thumb and middle finger.

So eventually, I had another look at the description and the images and saw that I was doing it wrong! I had put my thumb and index finger around the front of the right loop, rather than the back; once I started doing that, I started gripping the left loop with thumb and middle finger quite naturally, and the whole thing went much more smoothly.