I have been through these. I went through a phase of this happening for about 5 straight times of checking myself, and not fully becoming lucid. I have learned that my problem was that it had been a simple chore during my days and never gave it thorough consideration when I did my checks.
From then on, I changed my ways. Now, when I do reality checks, I consider the possibilities before going through with plugging my nose(My RC of choice). You and others, should REALLY consider what you are doing with these reality checks, and realize that even though you already think you're not dreaming, it is just as likely that you COULD be. I gauge the "reality" around me before I plug my nose, and tell myself how important this test will be, and if I can breathe, I AM LUCID, and have my own freedom. After the test, I think of exactly what just happened, again, and sort of replay it in my mind, and pretty much tell myself if I am dreaming or not, while I remember what just happened when I did this reality check.
I like to extend my reality checks for about ten seconds, and just allow myself to fully perceive averything. It is no longer a simple check while reading a book. I devote all my thoughts to the reality check at that point, and pay attention to it.
Sorry for my wordiness. (Add smile)

I have not yet failed a reality check after implementing this myself.