You are asking us obnoxiously, 'how the @^%# is thiz not a decap', as if the move has decent power for a decap and is actually hitting the target. That'd be reasonable, since I would also be very frustrated if I came across a move where it has decent power and yet I can't get a decap.

Then I watched the replay. You didn't even TOUCH Uke's head. How do you expect to decap him without touching it? Besides whiplash.

As for advice, hit Uke in the neck to get his head off. Shift + space to frame by frame for more accuracy. Atm, you are too close to take off his head with that kick.