My relationship with Yuna its maxed, but I think that when it increases any further, there's a bug and it drops to 20 and doesn't go back up.
Have you read Changelog since 0.3.4? If you'd do that, you'd know that dev locked relationship progress on 20 and 40 behind certain special interaction each, if you chat with yuna right before the night fun starts, she'll tell you exactly what she wants, of you do that, youll be able to progress further (20 has to do with cooking, 40 with running)
Cheers