Is there any way to increase my percentage?

Donna • SAHM, Happily married for 11+ years, 3 boys , +1 DD in Heaven. Hoping to push my luck for another girl.
I'm new to Glow. My fertile window is off I believe a few days, at least according to my physical symptoms. At Glows peak day I've only got a 22.5% chance of conceiving. Why is that? How can I increase my percentage?