Open central administration, click system settings, click manage services on server, check whether the Web Analytics Data Processing Service and Web Analytics Web Service are started on each server? If not, try to start them, then check the effect.
Second, click application management, click manage services applications, make sure web analytics service application is started. If not, please start it, then check the effect. Or you can provision a new Web Analytics Service Application.
For more information about Provisioning the Web Analytics Service Application on Microsoft SharePoint Server 2010, please refer to the following article:
Step-by-Step: Provisioning the Web Analytics Service Application on Microsoft SharePoint Server 2010 Beta
Introducing Web Analytics in SharePoint 2010