I know when I'm tired I have trouble finishing the pull. Have realized that it isn't only when I'm tired, though, I seem to have trouble more generally. I found this:
He takes a little while to get going...
But the basic point is that you need to have the ability to use your glutes to extend your leg behind midline without hyper-extending your spine. I tried the stretch and DAMN. Took 3 minutes before I could even get into the position for the stretch. After the stretch I tried kicking my leg back using glutes and not spine and things seemed much better.
Hopefully this is helpful for others since I've found that a lot of people seem to have trouble finishing the pull. Maybe partly because of this.