First off, what subject was the test in?
Second, review the goals you have set for yourself. Maybe your standards for yourself are too high.
Third, was this the first test in this subject?
Depending on what goals you have set for yourself, and what you want most out of the subject you are studying, it might not be YOU that is at fault here.
I am really good at learning and retaining concepts, but when it comes to testing for them, I don't always get the best scores. If this is a new class and a new instructor, give yourself some time to become acclimated to the testing.
Sometimes it is just getting used to the testing style that is the problem.
Did you study all at once, or did you space the study out over several days?
I find that a little each day is much more beneficial to me than attempting to get it all in at one time.
No matter what the situation is, a few weeks isn't really time enough to see all there is to see of a class, and your study skills might not be the best.
If you look at all these things, and find nothing wrong, then look at the subject matter. Is this something you could possibly study with a group? That might help, too.
I find that answering the questions that go with the chapter, and defining the key terms very helpful, and a lot of times I will make 3 x 5 cards with the key concepts on them.
Check all of this out, and see where you are at. At the very least, consider a tutor for that subject.
Hang in there!