the game has a number of bugs, a great many of which can be fixed or amiorated by installing the patches available from the telp patch links (and if you are able to install them, i have not been able to). but this particular problem is not one that i have seen any posts about. perhaps one of the posters with more time and experience can help.

if my memory serves, when you approach thorfinn (or any of the other promotion quest givers), the dialog presents a statement about seeking promotion, when you choose that item, he then asks if there are if there are any interested in, or ready for the promotion if you respond yes to that, he should then provide the quest information. if you accept the quests, you should then see a list of statements, one for each of your characters indicating which, if any, can chose to become a candidate for the promotion. selecting a candidate or indicating that you aren't ready ends the conversation.

i currently have a character working on a promotion from fighter to mercenary, when i speak with thorfinn now, he only provides a statement requesting information about the promotion (a refresher on what the quest is). i assume you wouldn't be asking the question here if you had already gotten the quests from thorfinn earlier, but...if that was the case you should see the quest listed in your active quest log.

with regards to your other characters:

priest is the highest promotion level in the initiate to healer to priest path. can't change from priest to anything.

druid is the highest promotion level in the initiate to healer to druid path. can't change druid to anything.

in order to create a mage you have to promote initiate to scholar, scholar to mage, quest giver for these promotions is in thjorgard.

your crusader can be promoted to either a paladin (lindesfarne monestary) or a ranger (frosgard).