Unsure if you have completed it yet but the first one false love happens when you give a girl you don't have high relation with a love potion i think its below 30 but am unsure. Relationship expert happens when you get all girls above 100 relationship points just give each girl a love potion to unlock the achievement. Hope this helps :)
If you are talking about the relationship of most girls being stuck at 50 there is gift items you can give them https://renryuu.miraheze.org/wiki/Items scroll down to the gift section.
