There's a few things. Most importantly is that seeing her (or any side character) counts as a "random event", which you can only have one of per day. So if you've had another event, such as something happening in the house in the morning, you won't see Library Girl. You can set the morning event probability to 0% in the guide to help with that. Turning hints on there will also show at the top if you've already seen an event or not.
You also need to have already fingered someone for her to appear, though if you're quite far through the game you've probably done that already.
At that point, if all of those are met, when you enter the library the game essentially flips a coin to see if she appears. You don't need to study, just enter the library, so if you're sure you've met all the criteria just go in and out of the library on the Saturday evening until she appears.
Let me know if she still doesn't show up and I'll look into it some more.
Best of luck! <3
