In terms of why you can't get the NPC quest, it's because of something weird in the game - this drove me crazy until I finally figured it out. Here's what's happening - he gives you a job that'll happen on April (I think) 21st. To stop you getting the job after the date has passed, the code must check that the month is before April and the date is before the 21st. However, because it checks them both at the same time, the quest is unavailable every month from the 21st on.
TLDR: You can only try and get that quest before the 21st of the month.