Say no to reality checks...
First off, I have never become lucid (personally) due to a reality check at random. I have always first felt that "I'm dreaming" feeling before following up with a RC. At that point I don't really need a RC, I just know I'm dreaming.
A second theory of mine is that, as you perform RC's throughout the day it forms a habit. With this habit also comes the "habit" of the RC's working since you are not dreaming. So I believe that if you RC just out of habit in a dream, odds are high that it will work, just as you would expect it to.
Other people may have more success with them, but they just don't work for me.