McAfee Signature Files Out of Date

I am using the McAffee Security Center software provided with my Dell system. Recently, I have encountered an issue in which the virus signature files are out of date.  I click on "update" or "fix" and neither of these correct the problem.  I have also tried using the McAfee "virtual technician", which appeared on my screen as though it was running for many hours, but never came back with any messages.  Any ideas?  Thanks.