As a creative being, your painter sim will need to be happy in order to feel like painting. Many times, if a sim's moods are low, they won't want to do anything, no matter how many times you try to get them to do something. For painters - as well as any sim - make sure that their moods and needs are constantly met and are constantly in the green.
Remember, sims need fun and play time too and while painting is a fun endeavor, sometimes you need to take your sim away from just painting all the time. Make sure they go out and do things or watch a little TV for some down time. Painting is fun, but doing it constantly without break will make your sim cranky and unhappy. So unless your sim has the loner trait (which can also help to lock them inside to paint), have them go out from time to time. Meet friends, family, and just hang out so that they are happy enough to paint.
If you have free will turned on, sometimes if your sim is truly happy, they may just paint just to paint and that's a good thing. That means they want to paint for the joy of painting (or perhaps because they know if they don't paint at least once a die, user God may smite them).
Make sure not to sell all of your paintings. As in the previous game, you can sell or hang a piece that your sim has painted in their home. This has the advantage of giving the 'nicely decorated' moodlet, which is a constant as long as your sim is in the house or in the room in which their awesome pieces are hanging.
Image content @ Carl's Sims 3 Guide