Line on HPT not getting darker....?

Hi everyone! I am thrilled to say I received my BFP on Dec 20 at 10 DPO. I am a POAS-aholic and have taken MANY tests since the first one. My question is--why is the line not getting any darker? It is there, for sure but no where near the practically burgundy control line. I will say that the tests I am using are cheapy because I go through so many (sorry DH) but I am nervous that HCG is not rising. Could this be? Thanks in advance!