Maybe it isn't meant to be a pinning technique?
I've been pinned pretty well by it...not that I'm necessarily a good measure of when someone is doing it correctly though. One method I've experienced was that when a really good pin has been done on me, my hips MUST go up in order to relieve some of the tension. I've also heard that using their arm (from pinned position) to cut across their head can help too. I tend to use my free arm to continue the elbow along as if I were going to pin it to the ground too, though being careful not to tear muscles (some folks are more limber than others).
Still...it's hard to give pointers by script and I'd agree your sensei or sempai might be able to see things more clearly. Ultimately, in my limited experience, if a pin isn't working, you don't have them over-extended enough somewhere.