First, I am by no means a FileStream expert. I am a DBA that was thrown a FileStream support ticket.
Problem, the developer ran the delete from the filestream table, but the files were still on the operating system disk taking up space.
When I first ran the sp_filestream_force_garbage_collection, I saw that I had a high number in the num_unprocessed_items column.
At that point I took a full backup of the database. Then I ran the sp_filestream_force_garbage_collection, and saw that number moved to the num_collected_items column.
At that point I took a log backup, and again reran the sp_filestream_force_garbage_collection. It took a couple of seconds to process, but then returned zeros for all the columns. Also the space was freed up on the drive.