Try these links to force the update manual.
Or, if that doesn’t work:
If that doesn't work, contact support, They will have you submit a sample for review.
I had a similar problem with a customer's computer. Where I couldn't update any anti-spyware program, but AVG was finding something with real-time monitoring, but not finding anything on a full scan. What AVG real-time monitoring was finding we couldn't find any information about the file on the web,