Glow app issues?

I have been using glow for a couple of years now. I usually have a 26 day cycle. Sometimes 28 but that is very rarely. I always put in the day that I start and the day that I end but no matter what glow never predicts my cycle correctly.

Is there a way to fix this? If not I may delete this app and use a different one. I like all of the other features of this app so I do not want to switch but because of my job I really need an accurate prediction instead of just having to figure it out for myself each month!