My 8 month old does not bark either ( with the exception of a night time bark at the possums maybe once a week. So I know he can bark) The rest of day he won't bark, even if someone knocks or comes to the front door and my smaller dogs are barking up a storm. He is also very mellow and if he wants something he does a very low whine.
My kids wanted to teach him to bark on cue, but how could we do that when he doesn't bark?
I know he's much slower to mature than some dogs, because my vet told me. (He's still having his adult teeth come out.) so that may have something to do with it. He also hasn't growled in 4 months. He only growled one time at my maltipoo when he was 16 weeks old.