I believe those things happen when you are trying to leave a rating on comics that are 'yours' - that is, the owner has an IP address that matches yours. I looked your account up and you share an IP with 2 other users, both people whose comics you've commented on.
Basically, the system thinks you are trying to vote for yourself, so it won't accept a numeric vote from you. So then, if you try and submit a comment that is a vote (invalid) and a blank box, it thinks you are basically submitting nothing.
The numeric votes don't have any impact on the comic, so the comic's owner isn't missing out by not having that number- they're just used as a sort of "easy" way to comment, where 5 is basically saying 'really great page!' without having to type it If you want to leave comments on those 2 comics though, you'll have to do it from a different location, or just put something in the text boxes. All other comics should work fine for you though.