Ive had this trouble as well. You need to be careful. If you move the stick Diagonally ti could screw you up. also the buttons are like HAir triggerd. So a light tap on the wrong one could mess you up as well.
I suggest you practice with the EX burst on with the EX-centric ruleset just to get the hang ofm the movements and button presses. I had trouble with this too. But as a lightning mainer You kinda need to learn it.(npt that im saying you main lightning.)
No problem~ I main lightning. So im learning all of her tricks~ but yeah. The game is seriously light to mess up on her EX burst. a quick simple tap of the Stick in the wrong direction can mess you up. still. Good luck~