The Symantec Endpoint Protection (SEP) definitions stop updating automatically. Running the command smc -stop and smc -start resolves the issue. The issue returns at random times.
From the SEP System Logs:
Error An update for Virus and Spyware Definitions Win64 failed to install. Error: Content update general
error (0xE0010001), DuResult: Failed to acquire LiveUpdate lock (39). LiveUpdate Manager
lulock.dat is not being released correctly, which causes all subsequent attempts to apply definitions to fail.
This problem is fixed in Symantec Endpoint Protection 12.1.6 MP8 and 14 MP2.
For information on how to obtain the latest build of Symantec Endpoint Protection, read Download the latest version of Symantec Endpoint Protection