I could book the PDX-NRT on united.com:

Portland, OR (PDX) to Tokyo, Japan (NRT) Thursday, Apr 6
Flight info Dates Misc Fares
United Airlines 372
Boeing 737-500 Apr 06 7:55 am depart PDX
Apr 06 9:40 am arrive SFO stops: Non-stop
Class: Coach
Fare Basis Code:
KXJP12I/UAF
Booking Class: K
Fare Rules
United Airlines 837
Boeing 747-400 Apr 06 11:30 am depart SFO
Apr 07 2:25 pm arrive NRT stops: Non-stop
Class: Coach
Fare Basis Code:
KXJP12I/UAF
Booking Class: K
Fare Rules
Tokyo, Japan (NRT) to Portland, OR (PDX) Wednesday, Apr 12
Flight info Dates Misc Fares
United Airlines 838
Boeing 747-400 Apr 12 5:55 pm depart NRT
Apr 12 11:05 am arrive SFO stops: Non-stop
Class: Coach
Fare Basis Code:
KXJP12I/UAF
Booking Class: K
Fare Rules
United Airlines 562
Boeing 737-500 Apr 12 12:48 pm depart SFO
Apr 12 2:34 pm arrive PDX stops: Non-stop
Class: Coach
Fare Basis Code:
KXJP12I/UAF
Booking Class: K
Fare Rules
Total Airfare (including taxes and charges): USD 382.86

This fare has a six day minimum stay, perhaps you weren't meeting that? A Fri-Sun departure will cost you another $50.