If you can find a W class ticket, there should be no reason why that can't be booked with the code or why it should vanish the moment you input the code..

if you find a ticket on W code without the code and the system doesn't let you book you should try their customer care and if they also don't book it, give them hell over it.. If they are purposely not letting people use their codes, even with availability in the correct code, that's plain unfair practice..