I had a LOT of trouble with this at first.
The problem isn't the timing being strict at all (at least not for me), it's that you have to make sure you don't press 1 or 2 (or 1+3) multiple times before doing the uu+2. Go into practice mode and check out your inputs when you try it. If you're getting b+1, 2, 1+3, 3 (uu2) it won't come out.
I hope this helps a little bit. Once I realized this was the problem for me, I started doing it 100% of the time.