When you run the report you have the option to filter the report for various categories or groups. The report pulls out the details only if the resource has met all the criteria. So in case you have setup the report filters for a particular group and then a category within the group and then a skill level, the report will show data only if all the three criteria is met. If the resource does not meet a single criteria then the report will exclude that resource. To see all the resources it is recommended to select all the criteria only for one filter and run it, this way it will pull the details for all the resources.