Why can't I join certain quests, even though I can create a game with them?
Solution 1:
Joining a public game on a specific quest is limited to characters who are in a specific level range, which depends on the quest (and the difficulty). This limit only exists for public games, not when creating a new game or when joining a friend.
For instance, joining a public game on the quest "The Doom in Wortham" on nightmare is limited to levels 35 to 41 (inclusive), as you can see in this screenshot (look for the red freehand circle):
That means that if you are outside that level range, you cannot join a public game which is currently on that quest.