I've had this bug, which it is, too while playing skyrim a couple years ago. I had to reload an older save but it seems they have fixed it so maybe you need to update your game?

Maybe siding with the empire or at least get to the beginning of their jagged crown request will help you out and *then* try getting the stormcloak quest.(I havent tried this myself)

Check out the bugs section on [this][1] page and the discussion on [this][2] page.


  [1]: http://www.uesp.net/wiki/Skyrim:The_Jagged_Crown_%28Stormcloaks%29
  [2]: http://www.gamefaqs.com/boards/615803-the-elder-scrolls-v-skyrim/61799984

That should help you out as much as possible and maybe you can add *your* findings afterwards.