In my lucid dreams, I often try to levitate and lift myself off the ground. I jump as high as I can, but all i do is stay in the air for about 3 seconds and fall back to the ground. Why can’t I fly?? 
Moved from General Lucidness. 
Im no expert but i think i have had this in a ND before. Try wearing an invisible jetpack or close your eyes, imagine you are weightless, and floating 100 feet above the ground and then open them and you should be in that state.
P.S. hope this helped!
You probably had doubts, or you didn’t have full control of the dream.
Believing it will work is essential. So try to rub your hands together and just let go. Don’t think about falling. Think about flying.
Also, flapping hands or going into superman pose helps for some people. Someone recently said that actually moving the ground, not yourself is their best method.
For me in ND, the trick seems to be to pretend that there is a breeze and that I am as light as paper and I am swept up high into the air. 
Check out this topic: [Different styles of flying)
Basically everything will work if you believe it will. If you have doubts though, it’s best to try a technique of which you think that it might work for you.
All the ideas in the link, I think, are great.
I’ve found that flying–like a lot of “dream magic”–is a matter of emotion. For me, it’s easier to fly well when I access emotional states of euphoria, awe, fun, curiosity, and enthusiasm. Believing is important, but so is the feeling behind believing. I have fallen out of the air from fear and doubt, and have risen higher and have sailed faster by building up an attitude of trust and fun.
Also, sometimes the “physics” of dreams seem out of the dreamer’s control. I’ve had a lucid dream where I’ve literally changed the directions of “up” and “down.” I’ve cast spells and changed into a cat, a dog, and a bird. And yet…I recently had a lucid dream where I tried to throw a rock up into the air and freeze it and it merely fell back down again. I would say I was very confident that I could control the rock. I believed I could. I was fully lucid and was excited about showing another dream character that it was a dream. Yet I failed. Why? Well, it would be easy to say that I “didn’t believe enough.” Some part of me doubted.
I don’t know. Possibly, the dream ego doesn’t always control everything within a dream. Possibly other parts of our personality are in control of dream physics even if we’re lucid.
I tend to agree with Robert Waggoner’s philosophy about dream control. He uses the analogy “A sailor doesn’t control the sea.” So, similarly, a lucid dreamer doesn’t control a dream; she merely directs her focus within it. Broad control is possible. I’ve experienced it. But maybe broad control is only possible within a certain subset of dreams–dreams in which only the ego is primarily involved.
Anyway, I got into all that to say maybe if you’re having trouble flying in a dream you should GO SOMEWHERE ELSE and then try to fly. Change dream locales and then try.
dreamosis’ locale advice is great. i find it nearly impossible to fly inside. obvious restriction being the roof. maybe your subconscious has barriers of it’s own?
I got this problem too… also when i jump in the air i sometimes turn around and fall on my head 
The Hitchhiker’s Guide to the Galaxy has this to say on the subject of flying. There is an art, it says, or, rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. Pick a nice day, it suggests, and try it. The first part is easy. All it requires is simply the ability to throw yourself forward with all your weight, and the willingness not to mind that it’s going to hurt. That is, it’s going to hurt if you fail to miss the ground. Most people fail to miss the ground and if they are really trying properly, the likelihood is that they will fail to miss it fairly hard.
Alright I’ll stop there. 
In all seriousness, you most definitely can fly. From what I’ve heard, believing you can is a big part of it. In waking life, think of a technique (e.g. superman) that you know is going to work in a lucid dream.
Not an expert at all (never flown in a dream).
Why not try jumping off a high place (i.e Grand Canyon, Burj_Khalifa or the IFC Tower) then float, glide, whatever…!
A friend actually told me the way he does it. He does it by taking some big jumps increasing the height each time, and when he’s high enough, he just starts to fly. So in my lucid dream, I actually came across my friend, and asked him what I should do.
He answered with: “Fly?” and I was like… yeah sure. So I tried his method , I took a big jump, but instead of making more jumps after it, when I was at the highest point, I just pushed myself off the air. I could just stop or turn wherever I wanted cause I could make the air under my feet solid, and the flying was really epic (backflips and stuff). you could try this maybe 
So when you’re at the highest point, just jump again.
You can’t fly because dreams are what I call “reflections” of reality. Think of the waking life as you looking into a mirror. The reflection on the other side is the dream world. You are so used to the laws of physics that it carries into your dreams. In your dreams, to fly, you have to believe. I know that sounds corny, but you have to expect to fly in dreams.To help you, you can use different things such as mantras or objects in the dream. For example, if you watch DBZ, go super saiyan and fly. If you like birds, grow wings and fly. Stephen LeBerge, Ph.D. says our dreams are made completely of what we expect. I recommend reading his book Exploring the World of Lucid Dreams.
I got the same problem and this problem is solved after reading this post. Thanks for the help friends.