Make sure you have the timing right, that you're not going to fast or too slow. Also, make your speed consistant, don't just spazz your fingers up and down the frets, try to make each press evenly timed with the rest.
I also had another question, does the same thing apply with the HO/POs as it does with the other regular notes? I'm talking about as long a you're not playing a chord, you can keep all of the lower fret buttons down.
I was thinking of trying to do it like that, for example, say the notes go from R to Y to B and back again(zig zag) I'll start by pressing down R, keeping it held down, press Y and then press B, let up off of B while still keeping the other two fret buttons held down, let up off of Y, hit the R note and then start all over again.