Why does my fertility window change.

As I log each and every day my fertile window was to be open until Sunday. Go to log in this morning and everything changed. Now says I am out of my fertile window. I don't understand. Yesterday according to glow should have been my highest day of conceiving. Now today says my highest day was the 18th. The one day I didn't have sex cause glow says it wasn't in the fertile window. No wonder why my husband and I are having a hard time. If it's not stress it I think this app doesn't have science and magic.

Does this happen to anyone else?

Feeling like I'm never going to give my husband a baby