Arnor isn't working

[Gniloi]

I conquered all the settlements, rebuilt house of kings, made an alliance with Gondor, and Aragorn is alive. Yet I have gotten no arnor message.

[Flinn]

mmm assuming you are speaking about Vanilla TATW, just to be sure I quote you the FAQs

Q: How do I play as Arnor? I can't find this faction in the campaign selection screen.
A: Arnor is an emergent faction that Eriador transforms into when certain conditions are met. These conditions are communicated to the player via the description of Aragorn's ancillary called Elendilmir which represents the crown of Arnorian kings. Simply mouse over the ancillary and read its tooltip.
These objectives are as follows: capture or hold: Hobbiton, Michel Delving, Longbottom, Bree, Staddle, Barrow-Downs, Amon Sûl, Annúminas, Fornost Erain, Athilin (new in 3.0!!!), Hoarwell, Tharbad, Coldfells.
Furthermore you need to establish an alliance with Gondor, build the House of Kings in Annúminas (only possible If you have upgraded this settlement into a large city) and finally Aragorn must be alive. If these conditions are met, then you will receive an event, which tells you can reform Arnor. Choose accept in case you want to play as Arnor.

if you are sure that you have done everything in the right way, either you missed the "reform arnor" messagge for some reason (and beware that it's not showing up again) or something in your installation is corrupted (pretty strange, TATW 3.2 is stable)

if you are using previuos versions that might be different, and if you are using any submod, ask directly in the submod thread.

[Macilrille]

The friendly Flinn said it all. Most likely you missed the Arnor Message.

Is Aragorn's Quest part of Vanilla? It is so long since I played it. If it is, you have to do that.

[Ngugi]

Is Aragorn's Quest part of Vanilla?

Nope, it's a MOS feature.