I have the same problem in falkreath. But I read on a wiki that it can because you have done the DB quest line where you kill the jarls housecarl, which will make him consider you as a foe.http://www.uesp.net/wiki/Skyrim:Thane_of_Falkreath
It is listed in the bug section.
Got the house i hjalmaarch though and I will get the falkreath house on another character.

I'm having the same problem. Can't become Thane of Falkreath. Starting to think its buggged out and i'll just have to start over....

You don't need to be the Thane. Or at least I didn't. I'm only 2/3's Thane.

For those saying you have to become thane, you dont.... I started by giving him the black briar mead from Riften, then I did the mission he sends you on to clear the bandits in knifepoint peak (think thats the name?). After I cleared the bandits and returned he told me to go to his steward, who gave me the land deed for Falkreath. I don't know where people are getting the idea you have to become thane first?

Cause you gotta be thane in most cities to buy a house.