Hi Sethu,
In the external defination for the target XSD please check if the occurrence of the root of the target XML is (0 to unbounded). Secondly test the message mapping with the payload uisng test tab. Check if it is prodducing correct output or not. In case its not producing correct output then there might have been changes in the mapping. Compare the same with other systems which is giving correct output.
Regards
Anupam