i have the same problem. when i talk to the steward there is no option to buy anything. i can buy morthal/dawnstar. and yes i am thane everywhere.
so i loaded a 50~ hours older savegame of the same character and suddenly i was able to buy all three plots. i have no clue what triggers this bug, but i would really like to build a house near falkreath on my current savegame -_-