I found a copy manual for your printer on the net. It appears that you should never have to specify the tray, so long as the printer has been informed as to what size paper should go with each tray, and the tray locking feature is not in effect. See the section on automatic tray selection starting on page 44. The menu system also seems to allow the manual selection of a particular tray - have you tried that?

You may want to specify the tray as 0 (zero), which may be the same as telling the printer to chose based on paper size.