Line progression okay?

So I think I finally got my second pink line! Its not a BFP yet, but I have been taking them two days apart and they are getting darker, but only a little tiny bit. I’m using the dollar store ‘Assured’ brand. My question; is it okay for the line to darken slowly? Are these tests less sensitive so that the line looks faint for a long time? How long should it take to get a nice big fat positive?

4/25: I could only see a shadow

4/27: vvfl

4/28: line visible in person but not in pic

4/30: positive!