Just so you know..... When you are doing these combos, you don't need to wait for the right moment such as Tekken to key in the buttons. You just tap them in the order you see at a moderate pace. Unless you see a "..." which is an air juggle combo you must wait until your opponent is at the right level to complete the combo. Otherwise, like I said, just hit the buttons as fast as you can in the order shown. Even if you finish typing in the combo a second or two before your character is done with the combo, you can just sit back an watch.
Hope this helps....