The Aes Sedai can't figure it out because, like you said, they think of it in terms of lifting. I think at some point early in the series Siuan says something about 'You can't lift yourself up with the power any more than you could pick yourself up off the ground with your arms' (or somthing like that.) But you can weave a platform of Air and then step up on it. You can weave a bridge between to objects or up to an object and walk up it. Flying should only require you to support yourself on air (like a platform) and move forward (more air) in a similar fashion.

At least, to me this makes sense.