SAPscript problem in update task

I am trying to print some labels based on a Transfer Order and using a standalone program everything is working fine. I have now moved the code into a User Exit ZXLTOU02 called when a TO is confirmed using transaction VL06C. The values in my print program basically disappear into thin air as they pass into the SAPscript. I have debugged the Update Task and the SAPscript to confirm this. The OPEN_FORM, WRITE_FORM, CLOSE_FORM all still work and return sy-subrc equals 0 bu the values aren't printed on the labels.

Any ideas ? Any tricks to pass the values into the SAPscript ? Is this because I'm in the Update task ?

Alternatively how do I configure a print program to work using Output Control and appear when a TO is confirmed using VL06C !?