The different archtypes have their own max interest they can have, if the type's max interest is 0, you can't get more than that. Like in my game, Gina's max is 80 right now ( hot and bothered sister ), Diane is 0 with restrained mother, Becky is 0 with popular sister, charlie is 42 right now with reawakened widow. I can go talk to Becky and gift her all the expensive jewelry, a pop-up will come on screen saying interest + 12, and it will stay say 0 on the character card. It's by design of the dev according to the archtype.
You can look at the picture I attached if you want, the top two functions calculate the increase in relationship, the bottom two are for the interest. The line to look at is the one with Mathf.Clamp in it. It's taking the variable ( temp.interest ), adding whatever number you earned in the interaction with the character ( num ), and then seeing if the result of the equation is between the min and max value set here ( min - 0, max - temp.Archtype.maxInterest ). If the number from the equation is higher or lower than the min/max, it will return the min/max instead.
It's easier to see on the relationship function, it's why I included it ( min = -100, max = 100). It's why you can't get more than 100 relationship with any character.
