Hi Dayong,
As per Modak - but also leading zeroes...
Keys are specific.
Also perhaps double-check your LPOR and FIND_BY_LPOR methods are correct and handle the incoming/outgoing key correctly.
One trick is to use SE24 test option to instantiate the class so you can double check what you are using for your LPOR value.
Secondly make sure the LPOR category and object type (i.e. classname) are filled in at design - not just runtime, i.e. default them via constants assigned to the attributes.
Missing this can make for some sneaky problems ...
Rgds,
Jocelyn