Wondfo false negative??

Hope
As of today im officially late. And no sign of AF comming what so ever! I have been SOMETIMES getting extremely faint positives on wondfo like this one
And its not getting any darker for the past few days. however i did take another brand pink dye test and there was a faint line but definitely more noticeable. 
Is it possible for wondfo to give false negative or hardly get darker??