Click on out of date report and check the client detail.
There are a number of reason for the client out of date
C Drive is full
Available content is corrupted.
Client not communicate with Server.
First check the client is online, if online try to run update contents.
Check the space avaialbility in c drive to update the definition
if disk space is available and client is normally communicate with Server then repair the client defintion
How to clear out definitions for a Symantec Endpoint Protection 12.1 client manually