Not Possible to Add and Change Reservation at the same time

Good Day Gurus,

          I'm currently developing a web based material reservation (mb21/mb22) .

         I'm using both bapi's of MB_RESERVATION_CHANGE_INTERNAL and BAPI_RESERVATION_CHANGE

          but both of them are returning a message of Not Possible to Add and Change Reservation at the same time

         whenever i tried to to add and update records in reservation,.

any solution?