Dragons not attacking isn't an issue. There are random encounters where they aren't hostile. I have a mix of both - some are hostile, some not (unless I attack them.)
Probably has to do with the fact that not all dragons are big meanies and some sympathetic toward mortals.
No this is a significant glitch which needs to be resolved.
as while i agree there are numerous cases where dragons simply just do their normal scouting..... I've found more and more frequently.. cases where the dragon will swoop in overhead.... notice you and trigger the music along with becoming a valid target...
However even while the dragon is being blasted with arrows and damage and even from other npcs.... the svcker keeps getting stuck in an endless loop of scouting and more often than not, just flies away after a period.
It's really weird, I've saved right before these occur and try many different ways to get the damn thing to either start attacking me or land or something. And i'm finding that sometimes you HAVE to run to a specific certain place at a certain time to trigger it to actually acknowledge you fully and attack as it was intended. It's like the scripts/AI are just malfunctioning completely.
Heres a good example of the total nonsense of a dragon attack gone horribly irritatingly wrong before i finally got it to land.
http://www.youtube.com/watch?v=5UqXwM5ws64
This isn't just happening once in awhile.... it's a battle in itself to try and get these dragons to attack. I've had word wall dragons do the same thing, run up to them, attack them, and then they just fly away sometimes.
Initially my first short play through, all was going well, dragons were spawning so often and attacking me directly that i thought that character was bugged and even without having any shouts i was up to 30+ dragon souls quickly, long lasting and deadly fights that i quite often wouldn't survive my first or 4th attempt in killing them. I even had 2 sometimes 3 dragons show up at the same time near the very start of that game..... i thought this was great...

.... unfortunately after creating the new one, i was lucky enough to have 1 dragon show up and actually manage to kill because they typically always seem to do the same thing as what is shown in the video above, come in with the apparent intention on eating me, only to fly around in repeated circles followed by leaving shortly after. I had 3 or 4 shouts to unlock for for each dragon i managed to finally kill. Dragons just seem very stupid..... very very stupid most recently.. with them randomly attacking other things and ignoring the player when they do finally manage to get their attention.