why aren't my lines getting darker :(

Kassie • A mommy of a little boy. Wife to a handsome man. Trying to build our family ❤️
I was so excited when I these test starting at 10 DPO... the darkest lines I've gotten were at 12 DPO. Here I am 16 DPO and they're still not getting any darker, should I try a different test?! I took a first response one and those lines came back faint as wel but there was color.