I've noticed that sometimes when somebody comments on a comic, the comment doesn't actually appear.

It's happened for one of my pages, so how do I see the invisible comment, if at all?

Those are empty ratings comments. There is nothing to read.

What the commenter does is give you a rating of 5 (generally) to show you they support and approve your comic.
Why they don't leave any textual content is really for them to say. Or not, in this case :)

I imagine that they just don't feel confident enough to come up with something appropriate. But that's just a guess... a lot of people are a bit shy. Sometimes though I think it's that they just can't think of anything that'd be better than the 5 to express their approval.

Not everyone is as verbose as me. ^_^

Ah, I see. ok then.

Sorry, I'm still familiarising myself with all the functions on DrunkDuck :P