Showcase has the ability to connect/disconnect from a discussion thread (the thread must be a discussion type 'discussion' (if the discussion type is set to anything else like a Thread Type (article, poll, resource, ams_article, etc etc etc), it can not be connected.
Go to the Showcase Item in question, click on the More options tool bar, click on "Change discussion thread" (This link will only be displayed if the viewing user has permission to change the discussion thread).
If there is no thread associated with the item, then the overlay will look like this. Simply cut and paste the URL to the discussion thread.
If the Showcase Item is currently associated with a discussion thread, then the dialog will look like this with 2 available actions (one to update/change and the other to disconnect.
Note: If the Showcase Item is saying that its connected, but the thread isn't showing that its connected, it could mean that the discussion thread has somehow lost its discussion type. What you can try is disconnect the thread from the item, then connect it again (which should fix a broken association).
Hope all that made sense?