Message view filtering messages by user role (or permission)?

I am using messages to log when a comment is made and when a forum post is made.
I have a view that shows the list of messages ("[Username] commented on [Postname]" and "[Username] created [postname]") in a block.

For logged in users, everything works fine.

The problem is that for anonymous users, the block does not show the "[Username] commented on [Postname]" messages - only the "Username CREATED postname" messages.

I am going in circles on this one and would appreciate any help or pointers as to why this might be happening? This has to be a permission problem somewhere, but so far I am unable to track it down - anonymous user has permission to View comments and View published content. Is there another permission they need?

Thanks for any help