I use an Xbox One controller with the circular D-pad. Maybe it doesn't do diagonals right? Could be. It's the motion, let off for half a second, then press fire. Sometimes. I'll try a SNES-to-USB this weekend. We'll see.
Honestly I struggle with inputting the hadouken in this game more than I typically do in X1 as well so I can say it's not just you. I can get it to work most of the time but it is pickier to be sure.
