getting unauthorized access when doing https


I am getting following error msg in TN console when i try to do pub.client.http from my flow service(Delivery Service)for few IS documents.The http server being used is the target side server.and its preffered protocol is primary HTTPS.Successful transactioons were also possible earlier using the same code and same credentials(uid/pwd).Even now also i am able to get successful transcns done.

Task 50u47t0v2cfjjq2a000002qq failed with no more retries available. Reason for failure - Delivery service for 50u47t0v2cfjjq2a000002qq failed with a status of fail and status message of Error during delivery: [ISC.0064.9314] Authorization Required: Unauthorized

Not sure though why few transactions are failing in a gap of around 20 days.Retriees are also set to 0.