My issue with this problem is now resolved.
The reason that my SEPM was not showing the correct definition dates was that the symantec endpoint protection service on the clients was set to start automatically by group policy.
SEP needs to be in full control of the service and having it controlled by group policy caused the reporting to fail.
Removing the group policy setting and a restart of the SEP service is all I needed to do. Within a couple of hours, all clients were showing as being up to date.