The type of Alerts displayed is based on the Scope/Alert type selected in the Alert template. Check that the correct Scope and Time period is selected.
Click on Manage Alert Templates on the Alerts screen.
An Alert template is applied to the selected Scope. Select the Active template of the desired Scope and click on the Edit icon under the “Actions” column.
Select the missing Alert Type and save the changes.
Note that the Site Scope is always preferred over the Organization Scope.