I have just reviewed the two tests you completed recently and the marking appears to be working properly. The correct answer is highlighted in green and the black tick indicates which answer you chose. If two answers were required for one question, then both answers must have been correctly selected for that answer to be marked as correct. If only one of the two answers was chosen, the answer will be marked as incorrect.
However, if you do feel there is still an issue to be investigated, please kindly let us know the test name and some of the question numbers which seem to demonstrate the problem.