I assume you're looking in the Diglett Cave at the Pewter entrance?
You might want to try beating Blue just in case. I don't remember having to beat Blue before I got Brock's number, but it's just a little possibility, specially seeing how bulbapedia says you have to. (I really don't think I did...)
That might be it 'cause he's the only Kanto trainer I've yet not beaten. I'd just like to rematch Brock, it would be great training for my Vileplume
Thanks!