Characters being unavailable at times is an intentional mechanic to lead the player away from fixating on one character. Every main character is meant to be progressed and fixating on one character will cause their story to be paused as you catch everyone else up.
Viewing post in Harem Hotel (18+) NEW UPDATE! comments
Yeah this happened with Code(my name for the android) on my end lmao, I'm personally more attracted to cuteness over sexiness so when code got unlocked I absolutely steamrolled her and Lin's stories, unfortunately once I hit somewhere around 30-40 I realized that I needed to level up Kali to progress Codes story. So for a while I was just playing catch-up lol. Also happened with Ashley since I'm not too fond of Maria(bit too freaky for me)