Been on the pill for years, stopped taking them around September time and started tracking periods on glow since December

I've had periods since not more than a couple of weeks apart...glow predicted I was due to come on 16 days ago

We've been TTC since I stopped taking pill

I've done many tests at all different times of the day all negatives, I'm not sure what's happening as to why I haven't come on 16days later

Any suggestions ?