The truth is a lot of these knock off companies have no idea what that stove will heat. If and I think they do use 1/2" fireboxes, well there is where a lot of your heat is going, straight up the stack. It takes 26% more wood to put the same # of btu into the water for a 1/2" firebox vs a 1/4" firebox.
So you are obviously getting your wood to burn, and a lot of it from the sounds of things, so the air thing is out the window. Your not melting snow which is another good indicator that your line is doing it's job, unless you just went so deep it's not showing up on the surface? So with that being said, your stove could be simply to small and not efficient enough, not efficient enough is the key word here, sometimes companies take a peak btu rating, them apply that to estimates of what spaces should require btu wise, one thing they never or rarely figure is that stoves rarely make max btu's and if they were, the wood would burn up in a few short hours.
There is a guy in my area who has an acme as well, your story sounds nearly identical to his. He bought one planning to heat 3 small homes, about 5,000 in total and he can't get burn times over 6-7 hours.