faint lines

I've taken a bunch of tests for the past 4 days and they've all been faint positives but don't seem to be getting any darker. One even looked lighter this morning then yesterday. And I had a negative digital test yesterday but it says accuracy is only 50% when you're 4 days out from your missed period. I plan on testing with the other digital test on Monday which is the day before my missed period but I'm concerned the tests aren't getting darker.