Know issue for xbox and ps3 users. Dragons wont land or hoover, they will just fly by, fly backwards or simply fly over in sircles without attacking the players or npc's.
However Skyrim runs great and without any problem on my mac.
happens to me about 50% of the time and i'm on pc. i think it's an issue that all versions got.
even though, it's not an issue for me. i just think, that not all dragons are bad.