Wake up in the most important part of the dream

Sometimes when i'm on the verge on knowing an important answer, i wake up. Ok, sometimes i have the unlucky coincidence of this happening because the alarme clock rings right when the dream is in the most important part. But there's those times where i'm asking a DC an important question and when he's about to give me the answer, the dream fades away. Or when i open a door to find what i'm looking for and i wake up.
Today it happened again. I'm alone in a classroom, fully lucid, i open my book and i write down:
"Dream, i ask you, what can i do to improve my life? Put the answer on the next page."
And as i'm about to turn the page, the dream fades away and i wake up. Damn!!!
It's like my subconscious is saying: "This is classified information." or "You're going too far now, Wake up."

Has this happen to you? And why this is happening?

Hasn't happened to me yet, but I think you answered your own question. It might be classified information, it might be that the subconscious doesn't know the answer (it certainly knows a lot, but it isn't quite omniscient) or you could have really bad luck. I'd say keep it up, but don't completely rely on dreams. Sometimes the answers are in waking life.

The first few times was probably just coincidence. But now you might be reinforcing it because you think it will happen. This is a very common problem across lucid dreaming.
If you say, "I wake up every time I get close to the answer," then you most likely will.
Perhaps you could try saying the opposite in mantra form. Something like, "I don't wake up until I want to."
Here is a guide on mantras.
Tutorial for Saying a Mantra

Another fix could be stabilization. The basic idea of stabilization (and what you will find at the core of almost every guide) is to engage all your senses within the dream. Anchor yourself.
In this way, stabilizing could really lengthen your dream time and many people swear by it.
Here are some links to helpful guides on stabilization.
Stabilization Fundamentals
Dream Stabilization and Clarity Tutorial
Tutorial for extending Lucid Dream Time. Hours of LD.

I hope you fix your problem soon and find the answers to all your questions.

Plathix