First check if it's reporting any errors in the logs. All the apps are under system Org, system Space.
Run cf target -o system -s system, then cf logs app-usage-server --recent to view app recent logs.
App-usage-server reads data from app_usage_service database. If app-usage-serveritself functions correctly but no data or incorrect data is returned, check if there are any errors in app-usage-scheduler and app-usage-worker logs.
App-usage-workerloads data from ccdb database / app_usage_events, service_usage_events tables, and inserts this data into app_usage_service database, app_events / service_events tables. Check if there is valid data in those tables. Follow the below steps for accessing ccdb and app_usage_service when using internal database for Tanzu Application Service:
$ bosh -d DEPLOYMENT ssh mysql/0
$ sudo -i
# mysql --defaults-file=/var/vcap/jobs/pxc-mysql/config/mylogin.cnf
mysql> use app_usage_service; ormysql> use ccdb;
Other jobs and tables in the app_usage_service database are executed or created based on data in app_events / service_events tables. If the data is not available in the two tables for any reasons, RESTful API will return an unexpected response.
If the problem can not be resolved with the above steps, collect the logs of all three apps and open a support ticket.