I'm having the same problem. Even 3 star maps I sometimes just choke and lose a beat or my finger just freezes.
I found that playing Multiplayer helps a bit, gives me the motivation I need to concentrate and the most important one:
Realize if you have something that might hinder your gameplay, as in real life stuff.
Did you not sleep enough last night/worked hard that day.
Do you have problems and the stress gets in the way of you concentrating on something else.
You did not warm up well enough, pretty important to have some maps ready where you warm-up.
These are some of the things that stop me from making perfect combo, but you NEED to play a lot, at some point you will get those combo's or even SS.
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