Mmkay, I was at Level 19, and I did "The Man Who Cried Wolf". As we all know, leveling up will trigger a courier to chase you down and deliver the letter from Falk Fire-Beard.

Except my next level was Level 20, where the courier chases you down to deliver the invite to the Mythic Dawn museum. I am now Level 23 and I have yet to have the courier deliver the letter from Falk.

Has anyone else had this problem?

I have heard that the trigger for the courier varies from character to character. Some people get it after one level, some after a few. Wait in Solitude for a while and the courier should find you.

have you tried talking to firebeard and see if he will act like you got the letter? How much in favor are you with the Solitude court?

Try picking Firebeards pocket he carries a copy of the letter once you level up.