I've found in the past that the paper.source enumerations Powerbuilder uses may not correspond with specific printer driver values.
A few years ago I used an evaluation program from Powerwhizz to find the driver specific values. We needed to print from Tray 3 on various printers which used different drivers. For HP 4100 and 4050 the value was 260 and with the HP Universal driver it was 261 so we used:
I can't find the Powerwhizz site anymore but if this is your problem there should be a way to find the specific printer driver tray values though some api calls.