If you label attachments and use the Attachments Macro to display attachments by label it will not display any attachments.

Steps to Reproduce:

1. Setup a Confluence 5.5.3.
2. Into a test space add a attachment.
3. Give a label to that attachment (test).
4. Now create a test page into the new space.
5. Add the attachments macro add test at the "Attachments labels" field:
   
6. Save.
7. Notice that the attachment added in step 2 will not show up.

Resolution

Upgrade to Confluence 5.6.3.