I am not a reiner, but if you just tap the horse, does that mean it is pressure that is applied for a milisec and then off? To me a tap might not be the best cue unless it's a string of taps. Why? Becasue it's the cessation of the cue that tells the horse it did the right thing and once it knows this, it should respond better, quicker next time.
If you tap and then stop , waiting for a reply from her, you have given the release when nothing happened to gain it.
I would apply solid pressure or almost annoying tapping that is like a stream of taps, and stop instantly when you get a step over. Pause, start again.
I use solid finger pressure, but now my horse is so sensitive that for the most part I just raise my fingers into a "tapping' place and kind of push on the air between us and cluck or say "over". I stop applying mental pressure the minute I even see him preparing to move over, like shifting his weight onto one leg so he can move the other.