We're working on this too!
I am having another issue with teaching it. If Thor already wants to bark at something, say another dog outside, he happily barks on command. If he's in a less energetic mood, he will only bark every two or three times I ask. Otherwise, he'll just murmer.
What I am working on is "shaping". If you are confident that Tyler knows he is supposed to make SOME kind of noise when you give the command, start only treating him for his louder noises. If he gives a real bark, give him a "jackpot" of four/five treats at once.
Thor of course has also concluded that he should just be barking more generally, so he's back to five alarm barks whenever my neighbors lift a finger.
I'm working on just getting the bark on command.