Only a few girls have their second evolve (called "blooming") available, and more girls are added only very slowly. Hopefully, Nutaku steps up the speed sometime soon, since we're currently getting bloomed forms at an even slower rate than they are added in the Japanese game at DMM.

Current list of girls who with a bloomed form available: https://harem-battle.club/wiki/Flowe...loomable-Units. Probably won't hurt to bookmark that page, and check about every other week whether a girl was added. Also, do read the scrollbars shortly before and after maintenance hours, those will also sometimes announce a new bloomed form.