I have 300 pages in the website and have included the piwik code in nearly all of them. The global statistics given by the provider indicate 13000 visites in july. But Piwik indicates only 19 !!! Where is the bug ???
How did you add the Piwik tracking code? Did you manually add your tracking code to 300 pages? Make sure Piwik tracking is working and the embedded code is valid.
By default DoNotTrack option is enabled in Piwik. Maybe those visitors also enabled DoNotTrack within their browser settings. That is why they are not tracked in Piwik. Go to Administrator > System > Privacy