Nothing wrong with zonal marking as a tactic, does my head in when you hear pundits acting like it's some kind of affront to "proper" defending. I mean how frequently do defenders lose the man they're marking, but you'll never hear them saying "man marking is a load of rubbish" every time that happens.
That said it needs practice and it needs the players to understand their roles and to command their zone. It's not surprising that it's not working great after just a handful of friendlies, it needs time to make it work.
I think putting our defensive problems down to zonal marking is a big red herring to be honest.