Hi, my name is Erin and I am a POAS-aholic. Now that that's out of the way, I'll say that I have spent alot (ALOT) of time doing this. Taking tests every day, every few days, til I'm out of internet cheapie tests and/or can't justify buying any more.
There are alot of factors here. Did you drink alot of water last night? Had you gone to the bathroom multiple times in the night? How does the darkness of the control line compare to your other tests? Sometimes there's just not as much/as dark dye in an individual test as another. Tests can give you an idea of what's going on with your body, and generally, yes, the line will get darker. Just remember that they don't give you a number, and cannot give you an accurate evaluation of your HCG levels.
I know so well the anxiety that comes with this. The practical side of me would tell you to NOT take anymore tests. If it were me, however, I'd probably wait a few hours without peeing, and then take another. Hugs and prayers to you.
Editing to add: What I mean to say is that I'd blame the test first. Do not automatically assume that your HCG levels are dropping based on one test. Remember, your body was made for this.