When you try to empty the Google Drive trash, it sometimes seems as if the files are not being deleted. When you reload the page, the files are still there, and the space isn’t freed.
The solution is really simple: just wait!
Google Drive takes a while to delete the files in the trash. If you check again in a few minutes, you will have more free space, and some or all of the files will be deleted. It can take up to a few hours to delete all the files.