Formatting Problem

I am writing out a series of exercises for guitar, and I'm having problems with the formatting of the page. I want to have six measures in each system, then a break. For some reason, it works out fine for the first couple groups of measures, but then there is a break after just five measures, even when I place the break after the sixth. I've tried to decrease the layout stretch, but it only works in some instances and not in others.