Debug CS Print Program

We are trying to debug our print program (defined in T390-ABAPNAME). When we set a session breakpoint and preview the output in IW52 we hit the breakpoint. However, when we press "Print/Fax" we do not get to our breakpoint - yet the program does run. We have tried activating system and update debugging and the breakpoint does not hit. Instead the program breaks in some ILOA UPDATE Function which does not lead to our program. How to debug print programs?