Should the line be darker by now?

Amber • Loving and learning! :)

I got such faint lines to begin with that I thought I'd use my remaining tests every couple days to see the line get darker. Here's the first one from January 7th, PM:

Next morning the 8th:

The 10th, AM:

And the 12th, AM:

I just thought it would be much darker on the last one if the HCG is supposed to double approximately every 48 hours and the sensitivity of these is 25 miu. Anyone have something similar and their pregnancies were fine?