From the wiki:
Note that if the Dragonborn has not yet chosen a side in the Civil War questline, then upon speaking with both General Tullius and Ulfric the first time, there will be no dialogue option to tell either about the peace conference. Instead, after mentioning Helgen, each will assume that the Dragonborn wants to join their side. Each will direct them to their assistant (either Legate Rikke or Galmar Stone-Fist), and in most cases they will then end the conversation. Approaching each one a second time will now present the option to tell them about the Greybeards and the conference.
After speaking to both parties, returning to Arngeir and speaking with him will allow the conference to begin.
So the question is, are you absolutely sure that you've invited both Ulfric and Tulius to the peace treaty or have you just spoke to them and they invited you to join their factions?
The complete walktrough can be seen here: