Call transaction ME33K

Dear Abapers,

I have a problem when calling transaction ME33K from a custom program I develop. I know that we can pass agreement number using SAP memory with parameter ID = 'VRT'.

This is the code to call transaction

      SET PARAMETER ID 'VRT' FIELD agreement_num.

      CALL TRANSACTION 'ME33K' AND SKIP FIRST SCREEN.

agreement_num is filled with agreeement number.

Before that I did same things to call ME23N and it works well.

Definitely I use same formula:

      SET PARAMETER ID 'BES' FIELD po_num.

      CALL TRANSACTION 'ME23N' AND SKIP FIRST SCREEN.

po_num is filled with PO number.

Could anybody help explain to me why this thing happened. Are there other any possible solution to call ME33K?

Thanks

Regards

Hadi