For the issues you think are missing from the report, perhaps those were before you had your approval process implemented? Or they were part of a different issue approval process? Try putting in more than 1 approval process in the filter. Or, to just grab all issues associated with an approval process, use 'is not blank' to grab all issues that are associated with an approval process.
Add a grouping for approval process name. You can also do a chart to show issues by approval process and by status (rejected, approved, etc.) to easily show what you're after.
If this helped you, please mark correct to help others : )