Sometimes it takes a bit, and it just happens. But sometimes, they just don't because of lack of appropriate info, a name or number isn't always enough it seems. Bear in mind, it's better to have the exact spelling of the Facebook contact name as well.
To join them, look at the contact in question, hit MENU / EDIT CONTACT, then MENU / JOIN. If info is whacky between the two, it may not offer the ability. Or may again, do it on its own when it syncs. Been there done that, found them connected later on. :-/
If they have an email or other phone on their Facebook that you don't already have in your contacts, enter that email into your google contact, that helps them connect the dots.
I've had people have an old number (disconnected or whatever) they left on their FB profile and seems that possibly hinders it occasionally as to auto linking. And of course, once linked, that old number disconnected will be in your contacts as well and you can't edit their FB info.