Why is my glow app completely off with my Cycle Chart??

I have had the glow app for about 2 year's now...& my chart is completely off when it comes to my cycle I only got lucky once or twice when it has actually been on track...I make sure I check in as much as possible but I raley get my cycle or ovulation day's the day it says I am suppose to... I'm wondering is it my cycle but I tend to have a normal 28 day cycle but its just difficult I'm debating if I should keep the app or not..Does this happened to anyone else or is it just me.