It could mean that you imagine a lot, of how flying in the sky feels or how getting up there means. Sometimes, dreams are just interpretations of how we think, imagine or feel or how we spend our days. They have no meanings as such.
Maybe you're wishing or thinking of going of and doing something new and exiting! The infrequent inability to fly means you feel you are ready, yet every once in a while have doubts. Nice way of thinking about it right?
I use to have flying dreams all the time when I was young adult. I read a book on dreams and it said flying means you want to escape from problems for a while and be less stressed. Falling back to earth is facing your problems.