Where is your walkthrough?
Link to a word document where I made a log/walkthrough if anyone needs/wants to use it. Note that it isn't professionally done or anything, so it might look like shit. However it should have the steps to unlock all scenes in the gallery. [Updated to have no issues getting the hair from Elizabeth ] https://1drv.ms/w/s!At-QRpkNEdPKpzb7_7DkTUgei0CM?e=Vh4Srp
here's the list of monster combinations as well. https://1drv.ms/w/s!At-QRpkNEdPKpziM-pV0duB2kWkc?e=h2kNOw
Just loaded up a new save to try and see if I can find the problem. I cant seem to find an issue, I was able to trigger the elf scene no problem within 3 days of the walkthrough day. You should have no problem as well as long as you have completed the dates with Cecilia, have started Rika's questline, and are playing on the most recent version of the game downloaded off their Patreon. If you became able to trigger the scene at a later date, let me know why/how so I can amend the walkthrough so people don't have problems in the future.