"My hypothesis is that it shows you who/what you need to see, rather than necessarily who you want to see. That would explain things like Harry spotting Crouch in GoF."
And also the 'Dissendium' thing, in the books, where Harry sees himself doing what he needs to on the map, which didn't happen until he needed to do them.

When Ron was carrying him in his pocket, Peter's name wasn't visible.
This seems applicable if we take movie canon, as the little nametag thingies may have overlapped each other.

Also, if Fred and George got the map in their first year ("nicked it off Filch" and gave it to Harry in their third year, then they only had it for one year, and perhaps during that time they never watched Ron on the map when he had Scabbers with him (remember he seems to run off quite a bit.)

And maybe Peter can make himself invisible to the map, at least while he's at Hogwarts, since he's part of the creation, and (disincluding the movie-verse) when he gets very excited towards the end of the book because he feels he's escaping he fails to do whatever it is he needs to or lets his defences relax; so up until then he'd not been marked on it.

This leads me along to train of thought such as Peter the Occulemens, and I've enough fics to write without adding another idea to my collection! *stops pondering*