Callback function module not giving old and new values

Hi Experts,

I have created an callback fucntion module.

I want to trigger this FM when there is a change in net value at header of an service order.

I have used business object as BUS2000116

Object as CUMULAT_H

Time Immediately

Event Atfer Change.

FM is although getting triggered correctly but i am not getting old and new values in the

REFERENCE(IV_STRVAL_OLD) TYPE  ANY OPTIONAL

REFERENCE(IV_STRVAL_NEW) TYPE  ANY OPTIONAL

parameters of this FM.

These parameters are coming empty.

Please help me here. How can i get values in old and new parameters of this callback FM.

Thanks & Regards,

Kapil Kumar