I could maybe help you if you can isolate what you think the problem might be. Also, what kinds of tests are you taking? Several things that I can think of that might help with any test:
Efficient test taking skills: do you go thru and leave the hard ones until
later--do all the easy ones first? This gives you confidence and leaves
your mind clearer to tackle the hard ones later. Also, it really helps on a timed test, b/c even if you don't have time to finish the test, at least you missed the ones you might have missed anyway, and got all the ones done that you know the answer to. I always do this, and I always do well on multiple choice tests--the kind I assume you're having trouble with? I have ADD so this was a survival skill for me, but it turns out that studies back me up on this.
Do you eliminate obvious wrong answers on multiple choice problems? Most of the time, at least one of the answers is clearly wrong, and often 2 of them are--eliminate those, and you have a 50-50 chance. On the remaining 2, pay close attn to the wording--there might be something in there that will give it away.
Do you look at key words in the prompt--ones like always, never, etc? And how are your reading skills? The better they are, the better of a test-taker you will be. It goes w/out saying that you should read the question carefully, but even though I'm a really good test taker, I will get tripped up at
least once per test. Sometimes, even if you don't know the answer, a key word will clue you in. An example: I'm studying for the spec. ed. praxis and here is a question on a practice test that I had no clue what it was except for the key word I bolded. "As an
intervention, response cost is best suited for which of the following purposes?" I had no idea at first what response cost was, but know what intervention is, so I got the right answer anyway, b/c only one answer had anything to do with intervention.
Do you have problems with inattention or suspect that you have any learning disabilities? Some people can be really smart but have ADD or LD, so will need extended time to take a test. Of course you need a dx to get this accommodation, but it might be worth pursuing if you think this might be a problem. I'm thinking this isn't your problem b/c you did well on the other types of tests, but other people will be reading this.
Whenever I have a big test to study for, and I'm learning the material on my own, I look up a lot of material on the internet, and try to find test questions. Sometimes you have to be careful of course, b/c I've found tests with totally wrong answers, or ones that were poorly worded. Also, if there are any practice tests that come with the tests you're taking, pay the money--it's well spent.
How are you at picking up on the "big ideas" in a book? When you study, do you look at the material in the boxes and do you look at the words that are bolded. They are bolded b/c they're important, so make sure you learn the vocabulary and study the stuff that's highlighted.
Now, if you could just reciprocate by telling me how you did so well on those other kinds of tests--the ones where you actually had to know the material?