I can only give you some support and advice in general terms.

- Keep trying, try not to be discouraged.

- For the "tricky questions" that you do not have an answer to, try to make a mental note and look it up after you are done with the interview. It will help you in the next interview.

- If there is a particular area you are having trouble answering questions on, try to get as much information as possible by reading articles/blogs, etc.

You probably knew all of this before.

Good luck!