I like this game, but it is a bit of a grind and with no clues as to how to get the H scenes, it's more like work than pleasure. A few hints or tips in the dialogue would be useful.
Check the phone. There are clues. But yes it is a bit hidden currently.
Suggestion : Adding the phone help as part of the intro and/or to the help section on the starting screen would likely help quite a bit
Since people systematically seem to miss it unless digging through the phone (which clearly they dont or simply miss this since it is not entirely obvious that the hints are there)
Open the phone, It is under show progress and interaction with girls, hover mouse over pictures that says not discovered yet.
And yes some dialogue hints like those when posing at pool (where they mention other bikini) would help too of course (gossip at dinner could be good place too for tricky hints and sexy situation tips)
Simply having the girls change to a choice of different lower level bikini, if you already have unlocked a higher level one for favor could also work, with the usual 30min game time delay while they change, but may be a bit forced in game terms (but very effective, since max revealing bikini would work as key for all as long as one has favor, the player then gets the scene matching the changed to bikini, since different bikini = different scene for some events)