Ovulation help

Inger
Can someone please help me with some understanding. 
​My cycles are pretty regular, according to my apps they are between 27-28 day cycles.. But according to the ovulation tests I've been taking (since cycle day 7) I'm not going to ovulate on day 14.. My ovulation tests haven't been getting progressively darker like I expected? I took one this morning and I could see the second line.. Took one this evening and now the lines disappeared? 
​I'm confused, can someone enlighten me please...
​Thank you 

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