How accurate are generic ovulation tests?

Abbey

So I've been using generic ovulation tests for a week and a half and I haven't yet gotten a positive response, even though Glow says this is my fertile week. I'm kinda starting to get concerned that these aren't very accurate even though I know they should be as accurate as the name brand ones. There should be a strong dark line when I'm ovulating, right? I'm starting to think it's worth the money to get the digital tests.