I recently had problems with agents not installing while migrating alot of machines from one Notification server to another.
I found this when looking at the agent install policies and found 70 or 80 machine without the default agents e.g. Inventory Plugin, Software Management Plugin, etc. after attempting to reinstall the agents and restarting them they were still not installing the agents.
I had a 5 minute blockout in place on the machines at system startup was stopping the agents from installing. After 10 minutes they should have tried again but it seems they couldnt find the information as they didnt seem to have they're codebase and had no idea what to do next.
Removing the blockouts in 'Settings > Symantec Management Agent > Settings > Symantec Management Agent Settings - Targeted' has resolved the issues on the machines.