I think that it's more that the potion has less effect on someone who appreciates you and even less on someone who loves you. Like Tsubasa who had the effects for a week while barely knowing Toudou but Itsumi and the prez having it hardly for half an hour and Toudou just maybe a bit more with...
Just wait until she sleeps with Kamisada's minion, and knowing Saya took her all the way to her home I doubt that she'll leave until the next day so the rizzler Toudou is at 4 gals in Bed now