Can the OP please give us the specific routes he flies and encounters this problem and how close to flight he books, and what fare is paid? Although UDU is in place I find even as a 1K on certain routes the advance upgrade doesn't clear until a day or two before the flight because UA is keeping F seats for sale (as Y-up purchases), or not until at the gate. If OP's buying last minute, why not buy Y-up?