The reason this will happen is your Users are not Editing, then updating the Comments section of the Event on or after the Event has taken place. To get the Event Counts to update, go to Activity Settings, and click on the button labeled "Schedule Events Update"Inline image
Once you've clicked on this button, nightly batch jobs will run and will update any/all Events where "Yesterday" = the Due Date. These records will get updated as "Completed Events". All Events where the "Due Date" is prior to yesterday will update when you click on “Get Initial Count” button. You will only have to click on the "Get initial Count" button once.
If you don't see the option on the Settings page, it means that you do not have the latest version of the Activity Counter. Only users with Activity Counter v1.601and above will see this option. To upgrade, using your standard Salesforce credentials, login to appexchange.com > Go to "My Accounts" > Activity Counter > Click on the "Upgrade Available" button.