We do only dream at certain times. Throughout the night, you will pass through different stages of sleep. The different stages will lead to different kinds of dreams. Some are more vivid than others.
It is often helpful to wake yourself up in the middle of the night (after about 4 or 5 hours of sleep), remind yourself of your intentions to have a lucid dream, then return to sleep. This is called a Wake Back To Bed (WBTB), and in my opinion, is the best way to increase your chances of lucidity.
Check out this tutorial, if you haven't already:
http://www.dreamviews.com/community/...ad.php?t=64626
If you keep doing what you are doing now, and incorporate a WBTB into the mix, you should be lucid in no time.