Hello JPAUMC,
I agree with SolarisMaestro.
Among the 15 clients, there is only 1 client which is facing the issue.
So concentrating more on 1 specific client with the issue would be a good option.
Repairing the Client from Add/Remove Programs is a Good and 1st Troubleshooting Option in your case.
There are couple of things, you need to look at in your case.
1) Is the Symantec Endpoint Protection client managed / Unmanaged?
2) Is the Client Machine connected to Internet?
3) What happens when you perform: START> RUN > luall.exe
4) Does the step 3 resolve the issue? If not, What Error code do you receive??
5) Work on the Article below:
Symantec Endpoint Protection: LiveUpdate Troubleshooting Flowchart
Hope that helps!!