REPORTS-Participant List 4.6.10
I've had the same problem - it's a bug in the current version of the
Participant list Count Report which should be fixed in the next release. The bug report is here: https://issues.civicrm.org/jira/browse/CRM-17485

It's the participant role filter that is causing the problem. If you de-select the participant role, you should see all the participants. Depending on what else you need from the report, you might be better off using the 'Event Participant Report (List)' report.