Freeing disk space after dropped database
Try sudo lsof| grep deleted and check if any PostgreSQL process appears. This command looks for files which have been deleted but its file descriptors are still open by any process. Another side effect is that df -h and du -sh / differs. This is because du looks at the file system and sum up the size of all files, and df looks at the physical device. 

I have just had an issue with a database which didn't free any space after a DROP table and that was the cause.

The only solution I know is to restart the database. Maybe you can try to send a reload (SIGHUP).