Sounds like they are already deleted. try right clicking the folder and selecting compact. This will force an IMAP expunge command to the server and finally delete the already deleted messages.

Or you could go to account setting > server settings for the account and change the setting for deleting message from just mark it a deleted to move to this folder and select your version of the trash folder. But expunging is a good way to ensure you do it occasionally instead of forgetting it.