Hi Michael,
What we realized was .. the main screen you have check box to mark all the options.. So in your BDC recording first you mark the required check box and then execute the screen logic. What we realized was that suppose any other user logs on to the transaction MK02 and records or does something with the selection of check box as per his requirement what he had done gets stored and affects the BDC simulation which is run immediately after him. So what we did we picked all the names of check box other than ours and set it to blank in the BDC program and set 'X' to the option which we required. i,e Returns Vendor Flag only. Hope i am more precise and clear now. Hope this helps.
In our case it worked fine after this rectification.
Sorry for the delayed response.
Thanking You.
Regards,
Navin.