I have a postgres (v10) database hosted on an Amazon AWS instance (which was running very low on 'Free Storage Space'). It had a 50GB table on it which I then dropped (pgAdmin - right click, delete/drop table).
I then checked the monitoring page on our AWS console for this instance. The Free Storage Space graph did not show a 50GB jump in free space. It remained static, even after waiting a while, refreshing the page etc.
Did dropping the 50GB table actually do anything at all?
Close pgAdmin or end the transactions on the database in the 'Dashboard' tab of pgAdmin. This will cut the transactions putting a lock on the database, and the free space will appear.