Let me help you out, bud. I had the exact same issue, and I spent a long time searching the forum for a solution to almost no avail.
Basically, you have to let the timer run out, BUT you must also be dancing with Liandra when the timer runs out. If you start the Party at 6pm, you get 5 actions in the Party. So select "Dance Alone" 4 times in a row. Liandra will choose to dance with Lyvia at some point, so just choose to dance with both of them for your 5th option. The Party will end, the scene will play, and the option to cum should be there. Oh, and I did this without Tia being present, so I dunno how it goes down when she's there, but it's probably the same. Just be sure to be dancing WITH Liandra on the last (5th) choice, not just having danced with her last.