The current requirement for her to accept underwear as gift is romance level > 200.
When you "observe" Fuyuka and it show "She is in love with you..." means the romance level is above 200.
The 1-6 Save I sent you has 253 romance but it still doesn't work. Obverse says "She purposely sit like that" edit. Cheated romance to 999 and its still the same.
