Sometimes my RCs are turning to be negative when I actually am dreaming.
Example: I have bad vision with my left eye, so I use that as a RC.
But even in my dreams, when I do that RC I have bad vision with my left eye.
I guess that’s because I have an expected outcome on my RC. Like I expect to have bad vision.
You should always use 2 or 3 RCs, because they do fail like that.
RCs are a bit tricky. Best approach is if you think you might be dreaming YOU ARE.
I need to wear glasses IWL but in most dreams I don’t wear glasses. So I thought as an RC I will check if I am wearing glasses. For several months I would forget about this RC while dreaming. Then my NDs started to have me wearing glasses 
Usually I now KNOW I am dreaming. But if I have use an RC I use “hop in air”. I jump straight up and see if I float. So far this RC has never given me false negative.