Post the sylink file from one of the affected client, How many content revisions you have configured in SEPM, if they are outdated from FEB its quite possible that tey are requresting full update from SEPM and SEPM not able to provide that, check if you have any band width restriction in your network
When will a client download a full definition set from a Symantec Endpoint Protection Manager or Group Update Provider?
http://www.symantec.com/business/support/index?page=content&id=TECH131528