as long as you have legitimate pins they have to trade with you, unless they have a green lanyard (it's green at disney world, not sure about disney land) which means they only trade with children 12 and under.
i've never had any really reluctant to trade with me, i've gotten things like "oh darn i liked that one" before though. and a couple cast members told me they have a certain number on the front of they're pouches and if they get one they like they'll swap it out for a different one from inside their pouch.
however i don't know if there is a required amount of pins they have to have out though, i saw some cast members with 9 or so pins on their pouches/lanyards, some with 4/5 and some with only like 3