My predications keep bouncing

I know that glows uses what I log and makes an educated guess off that. But my fertility window keeps bouncing around. Every month it changes. I figured it's just still figuring me out so I haven't minded too much. But this last window (that has already passed) was accurate, I had all good fertility signs during that window time. But now it moved to the week after where i had no fertility signs. So that doesn't seem like an educated guess it seems like a bug.