Hi! There's a couple of things that could be going on here, but the good news is that it's probably not a bug.
If you can't find the link to the Extra Story in the Extras menu, there's two things to try:
- Be sure you've reached a good or neutral ending for all four love interests (that's ending 4 or 5, 14 or 15, 19 or 20, and ending 10.)
- If you have, please try going through the game credits through any of those endings
The game checks that you've finished the required endings to unlock the Extra Story during the credits (it kept things simple when I was programming!), so if you went back to the main menu after getting all the required endings without going through the credits first it may not have completed the check.
Hopefully that helps!