
Sometimes one is not able to succesfully delete the Trash. Often because a file is still locked or used be another process and often files removed from the Dropbox are very balky.
Even if you only know it from your last vacation - sometimes the Terminal is your friend. Open it and type:
sudo rm
and then drag the file(s) you want to delete onto the terminal window (of course you can also directly type the path) and don’t forget to hit Return…