Amazon package "missing"?

Last Wednesday, I ordered a book that Amazon said was definitely due to arrive today. So far, so good. But when I checked the shipment status today, it suddenly said the package had already been delivered on Thursday, and the confirmation had been signed.

So I go to my dorm's mailroom, but they tell me they don't have a package for me. And when it comes to the tracking number, they just tell me that there's no package there, which is a bit contradictory to the delivery. I didn't enter it incorrectly, because I copied the number into the tracker.

Since we're a very large property that encompasses several house numbers and is well-known in the area, it definitely won't have ended up with a neighbor. Yeah, what do I do now?