Some databases in Availability Group are not showing up in SQL Performance Investigator (PI) dashboard under the primary node. The databases can be seen on the secondary node only.
Databases appear in SQL PI only if there have been at least one execution in the selected time frame. In this case the database on the secondary node it busy due to the synchronization and therefore it appear in Foglight. But one the primary node there is no traffic.
Please select another time frame, for example 4 hours or 24 hours and check it the databases appear now. Or ran some SQL statements, wait until Foglight collected the metrics and you will see the databases appearing in PI.