I've reached lv 40 and so far three dragons have circled overhead without landing. They hover far up screeching, dodging and weaving so I can't even get an arrow into them. I have gathered many dragon souls so this is something new.
I think you should ask this is the Hints and Spoilers forum. I have an idea:
Spoiler
I think the Teacher at the Throat of the World said that having the Dragonrend Shout causes Dragon's to fear you...do you have that shout? If so then I think all you need to do is use it.