Hmmm... Well, on Day 5, did you ask Jin if you wanted to visit where he lives? It's a detail quite a few people glossed over the first few times.
Your choices before this day matters. When visiting the construction area, be sure to ask on checking out Jin's house. If Jin says "ask Leo" instead of a reincheck, this means you did not hang out with him on prev days or did not choose the right responses. This is already a sign that you are locked out of his route. His route is purposefully made a bit harder to reach because of his mannerisms. Make sure to save often so you can reload if you ever make a mistake.
And in case some wonder, your dateable is not available all the time. Some days your dateable will not be available at all, because they are busy.