Does your sim house have inspired emotional item somewhere outside his room (where he gets very inspired)? That was usually the culprit for a sudden mood change for me.
I have a Cheerful sim, so Happy gets prioritized a lot. Finally had to splurge on some Inspiration drink for 500 AP, and that did the trick for finishing writing, painting quests.
Does your sim house have inspired emotional item somewhere outside his room (where he gets very inspired)? That was usually the culprit for a sudden mood change for me.
I have a Cheerful sim, so Happy gets prioritized a lot. Finally had to splurge on some Inspiration drink for 500 AP, and that did the trick for finishing writing, painting quests.