Its not possible to report site-wide on individual activity completion, you have to visit the individual reports within the course. 

This is basically what's happening: - Course completion is usually based on activity completion. The Certification completion (or when the user becomes certified) is usually based on the course (of courses in the cert) completion, according to the certification settings.

When the user has certified there there will come a period when they have to recertify; the Course completion is archived & the activity completions are reset.

In order to understand when activities have been completed in a Certification you need to understand what the course completion arrangements were & when the courses were complete.

To do this its probably best to use the Certification overview report source, there are specific course completion metrics available. If there is just one activity per course is should be pretty straightforward to pull the results, you also could use a straight Course completion report. 

If you enable the Program completion editor under Site Admin>Advanced features then you can use the 'Edit completion records' functionality under the Certification 'Completion' tab for a more fine grained view of users' individual completion records.

In short - you can't report directly on activity completion, but you can for the courses which rely on them for their completion.

hope this is useful,

cheers,

George.