WCF doesn't like me adding to lists...

I have to make an uno game that supports multiplayer with WCF for school...

I have a player class that has a list of card classes.

I have a function called dealHand, it adds Cards to the hand list...

I also have a callback function for updating my wpf gui...

For whatever reason if when the gui calls dealhand and I try to use myList.add(new Card()) then the updategui function in my wpf client doesn't fire...

But if I take a seperate method called makeHand which returns a list of cards, and just do myList = makeHand() then that works fine...

Never been so baffled/cursed so much at code before... Is there something about Lists that WCF doesn't like? Seriously... was weird...

Hopefully that was somewhat coherent...