Hi Sunil,
Can you kindly check/confirm the following
1. Are you using the standard integration for service items or have you changed anything in Sourcing or PI ?
2. Open the MA that is published - go to the header and check if the Document links section has an OA number there.
3. If the answer to 1 is NO (it is all standard) and the answer to 2 is "you don't see the OA number' then this needs further investigation.. Please open an OSS message and provide the XML payloads that are going from PI to ERP..
Alternately, You can try debugging the BBP_ES_ANALYZE to see what is going on with the OA numbers..
Regards
Prasad