Will the agent restart from command line using
/opt/imperva/ragent/bin/rainit start or does it always require a reboot.
Have you ran the
which_agent script to make sure you're on the current agent version for the OS?
I have seen odd problems when the server gets upgraded without upgrading the agent.
Can you check the logs for the agent, look at /opt/imperva/ragent/etc/logs/ragent/ragent_log.htm
It may show you what is happening to the agent.