Well the only times I have trouble flying are when I'm not absolutely positive I'm dreaming at the moment. For example, maybe there are people watching or something and I suddenly have the notion that this might be a dream, so I initially try and fly, but I can't right away because I'm not positive it's my dream. (So then I'll do some RC's etc)
The best feeling I can equate it too is walking down the street in the waking world, chanting to yourself that gravity doesn't exist if you aren't quite sure your dreaming yet. It just wouldn't work...
Usually after a few seconds of RC's though it's business as usual, I'll spring 30 ft just to get the feel, then just *pow* into the sky.