some times the quests bug out i gave up playing skyrim because of it with over 130 hours played. i did all the main missions but i had about a dozen side missions i couldnt complete because of bugs.
all i can recommend is you restart from a save previous to starting that mission and trying again.
or you can use the command console to complete it for you... but 1s you do that it can make things worse as you will be tempted to do it for difficult missions.
Had the same probs the first time around. Playing it the second time, what I do is make sure I don't progress too much on the main quest and keep having fun with the open world and the ga-zillion side quests!
Worked for me so far, no bugs yet....touchwood!