It's as Carlthungis said.

Patience is one of the most important, if not the most important thing about lucid dreaming. Keep trying very actively but never get discouraged. Always hope to have a lucid dream, but never expect you will surely have one. That will only discourage you. It seems to me like you're doing fine, and you will probably have a LD pretty soon.

Also, autosuggestion might help you. You might already be doing this, but what I'm talking about it telling yourself to try and recognize the dream state and become lucid. When doing this, however, it is also important not to tell yourself something like: "I must have a lucid dream tonight!". This will only discourage you. Instead, you might try something along the lines of: "Tonight, or in the near future, I will have a lucid dream".

Finally, the fact that you're studying for exams means you head can't be on lucid dreaming all the time. If you don't succeed during your exams, you surely will after they are over.

Whatever you do, don't worry about it. Know you're going to succeed sooner or later.