For those, like me (in BfA), returning to complete Legion class order hall quest to obtain your mount, pets, etc and find you have reached an apparent dead end (meaning you've done all the quests you can find in your order hall and on the Broken Shore), simply means you missed a quest somewhere. Trying to find what quest you missed can be super difficult and super annoying.
You can try contacting Bliz support, but they're not super helpful (or give vague answers) and can take several days to respond.
You can also try flying around Legion doing random quests you find, but that really, really sucks
Or... What I used to find my missing quests were 2 things:
- I linked my wowhead account profile to the character I was working on (you can do that here: https://www.wowhead.com/account#battlenet). This is useful because after linking to your account to a specific character, Wowhead then provides you with a "Quests" tab where you can lookup specific quests to see if you have completed them or not ON THAT CHARACTER. (note: you have to exactly type in the quest title including any punctuation - like apostrophes)
- However, the above isnt that useful without a clear quest chain listing. After spending A LOT of time trying to find a complete listing of the Rogue quest chain for "A Hero's Weapon", I found that entire chain on Wowpedia (gamepedia.com). Using the titles from the quest listing combined with the followers you have (or dont have) added to your Mission table you can narrow down approximately where in the quest chain you left off and search for those quest titles on your wowhead linked profile (see #1 above)
Ta-dah! Missing quest found and you can continue onward...