PSCD Event 0600

I have a requirement to raise an exception (and send to Clarification Worklist) if I have an interest disbursement and there is no SSN or tax id on the Business Partner record.

I am looking at event 0600 which I expect to be triggered when I run transaction FPY1 (Payment Run). I have put breakpoints when this exit is called and on the function modules that calls this statement (FKK_FPYM_START_PAYMENT_PROGRAM) but my breakpoints are not being triggered. In fact when I step through the FPY1 program (SAPLFKKACTIV2) although it does briefly go into SAPLFKPMASS, it does not call FM FKK_FPYM_START_PAYMENT_PROGRAM.

Any suggestions would be greatfully appreciated