I think the reason I can do only one very intense set and then I'm done is because I'm used to do very "efficient" reps. By that I mean "CNS-driven" and not "muscle-driven" reps. Basically, low TUT and high speed in order to finish the rep as quick as possible and with as little muscle breakdown as possible. That's how I'm wired in general.
Once I get to actually tire my weak muscles, no matter how hard the CNS tries, I won't get another high rep set. I will be able to do low rep sets where the CNS is king, but moving into the muscle realm of higher rep ranges... good luck with that one!