Fix my fertile days

Mere • Waitin on Our Special Rainbow 🌈 Baby 👶🏾

So I changed my period to every 27 days but it won't fix the days I'm suppose to be fertile. I'm not sure why it won't change but I know it's wrong cause I have other apps on my phone to help me track my fertile days as well. If it can't change then there's no need for me to have the app