Hi
You can try to debug and find out why it is not getting called from fiori.
Also, while executing the approve action from fiori the method called is "/IWBEP/IF_MGW_APPL_SRV_RUNTIME~EXECUTE_ACTION" of class CL_GBAPP_APV_PO_RDP.
It has an enhancement point specified. "GBAPP_APV_PO". You can try to use this for your custom code.
Regards,
Sarbjeet Singh