I have the exact same problem...I'm starting to think it's a bug. I originally went back a few days to reset some things on my island, talked to blathers about art, then skipped back to the current day not realizing redd was supposed to show up the next day. I tried time traveling forward into may as well, didn't get him. I tried going back, talking to blathers again, then forward one day at a time up to today (talking to blathers every day just in case), still nothing. I'm going to try and give blathers a real painting if i can get one from a friends island and see if that resets it. let us know if you try something like that and it works! i have no idea when I'll be able to try it, lol