Hard to tell from the photos, but do the angle of the roosts position birds on top to poop on birds sitting at the bottom? When you said the roosts were 18" apart, I was thinking horizontally... have you looked at the birds when roosting to make sure no one from one roost can easily reach over to another one on the roost?
I just noticed you mentioned this wasn't the original roosting set up. What was the original set up like? The change might've been enough to throw her off that she felt "safer" picking a new spot.
I just noticed you mentioned this wasn't the original roosting set up. What was the original set up like? The change might've been enough to throw her off that she felt "safer" picking a new spot.