I had a similar issue when I first shifted over to Ultimax. I went into training mode and tried it for a while, and it read the inputs as being correct but the move wouldn't come out. It seemed to me that the timing of the input, particularly whatever button press accompanies the 236236, is stricter than in either P4A or BB.
I found that it helped with my timing if I tried to press the accompanying button right as I hit the final 6. It just takes a little practice to get used to, but if you mess around in training mode for a while you should be able to get it.
Hope this helps!