Complete_VSS operation may fail if snapshot takes more than 45 minutes to complete, due to exceeding the default VSS timeout:
In order to verify this, you should check the Agent's logs - VSS script should fail with return code "3" immediately after receiving the request to perform "complete_vss" from CPM Server:
2017-07-30 02:48:08,841:[7156] INFO: execute_operation_synchronous(remote_agent.pyo:555) Got new operation. Op_id: complete_vss, vss_process_id: <subprocess.Popen object at 0x00D83910>, all_snapshots_succeeded True
2017-07-30 02:48:08,841:[7156] INFO: run_vss_stage_3(worker_functions.pyo:567) Starting on policy PolicyName
2017-07-30 02:48:08,841:[7156] ERROR: run_vss_stage_3(worker_functions.pyo:579) Process of the vss script exited with return code: 3. Failing