Unable to delete files sometimes

I have a folder for downloaded media from SabNZB, after I copy a folder containing media files from the unRAID folder to a folder on my Mac, I go to delete the folder and often get this message and I am unable to delete the folder for whatever reason. Sometimes I can delete some or all of the contents of the folder I am trying to delete but not the folder itself. Oddly I am able to rename the folder, so I usually rename it 'old' and after some period of time, I am able to delete it. It's totally random, sometimes it's one folder or several. It also not because I have a media file open in a player such as VLC player either, or any other app.