Digis work differently, it's the shade of blue they're measuring, so if it isn't quite the right shade it won't be positive. They're weird. That's why so many become confused when they pull them apart and see all three lines there with a 'not pregnant' result. If you have had lines on all regular line tests with varying sensitivities but only used a single digi, it's possible the digi may be faulty so I would try another.
I would also agree with everyone else and have bloods done. What day did you think you ovulate on? Are you tracking temps? Fertility Friend I believe got it wrong not long ago for me as I had a positive on what it said was 5dpo, but using research detector it put me at 7dpo which was more realistic. I also usually ovulate around CD11 or CD12 but last cycle I ovulated CD9 which I haven't had happen a single time since tracking so definitely possible you ovulated earlier than you thought