On my first install of piwik it was not collecting any data. In my first look at the source I noticed there was no php end taf (?>) at the end of piwik/piwik.php. Is there a reason for this? I added one but ultimately that was not my problem and I had to change some other things in my configuration, so I do not know if that was part of the issue or not. I marked this low priority as no one else seems to have an issue with it.
the closing php tag is not necessary, it's often remove to avoid possible "header already sent" errors