Aunt flows a day late


Hello all, I am curious if this happens to other women. Jan we I started this app tracking my period and such. Well my period is very normal and always on the date my app says. Well Jan my period was 7 days late but she ended up showing up which then messed up my cycle. But then for the next five months it was on time. Now my period was supposed to show yesterday and she hasn't ! But these past 5 months it's been consistent and spot on with my app tracker. Is it possible this will keep happening every six months and does anyone know why?