Search on child metas is different (and I really don't like it).
It only indexes "recent" users - so if someone hasn't participated on meta in a while, they won't show up, even if they have an account and have been very active on meta in the past.
This is the /user search results for my username on Interpersonal Skills Meta:
But I was a moderator there for a year and have over 130 posts on Meta IPS. I do show up on the voters and editors tabs but that's not what most people are going to search - there really should be a way to search all meta users.
This annoys me to no end but I haven't really been annoyed enough to do anything about it. In general, I do what you do - search for the user on the main site and go to meta from there.
/-1
when you're onsite.meta.stackexchange/users