Sorry if I am being a bit dense, but what is a parcel warehouse?
If, for example, you have sent with Evri, then this is an issue with them and there is not anything Ebay can do.
Some methods of posting only show drop off at the point you send your parcel and then delivery.
Was the buyer in the UK?
As advised, if the latest estimated delivery date has passed, the best way forward is for the buyer to open an 'item not received' case.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)