I don't have it with me, so I'm going from memory here...
But I'm 99% sure that what's happening here is something sticking on the film back itself. The problem is not present 100% of the time...maybe four shots out of ten it will stick on this particular back. I can see the thing on the body trying to "poke" the little trigger (that I end up triggering manually when it sticks). Its trying. Its just not getting it done. And I have another back that works 100% of the time.
With all that in mind, does a squirt of lighter fluid into the hole on the back sound like the right approach?