Some of the quests, specially some parts of jobquests require you to SOLO the quest, meaning you can't be in a party. If you party someone you won't be able to complete the quest. This is the only issue I can come up with, let me know here if that solves your problem.
This is bull****, you can do jobquest at any lvl. there was a time in which you couldn't after 115 because quests reset after you go past 99, they however fixed this when they revamped the payed service to race change, which requires people to re-do the jobquest. I am not aware of any issues since (this has been fixed like 2+ years ago, I was involved with that back then so don't try to call bs on me lol)