You are not backing up your reality checks with...uh....reality. The whole purpose of RCs is to differentiate dreams from reality. But if in real life, you do a reality check and don't concentrate on actually realizing it's a dream, you will do the same thing in dreams, you will do a reality check and think, " oh, I can still breath! No, it's just my imagination." and thus you wont become lucid. So be sure that when you do a RC, back it up with lots of awareness.