Guns and Things, Blackie AB.
Sells target airguns, among other things.
Probably worth a try.
Used to be a RCMP guy (retired, maybe) by the name of Jerry Tynowski, or something very like that, who was an airgun guy in Moose Jaw a few years back. May still be around, and may be worth calling. Been about 10 years, so he may be long gone, but I knew guys there that went as long without getting out of that town, so you never know.
Probably something complicated, like a bit of varnished up oil holding the valve open. Try a drop of oil at the intake port of the CO2 chamber, and dry fire it a half dozen times, before trying more CO2.
The TAU's usually come with a full set of seals and instructions for maintenance. Yours not? Or you just not feeling like the adventure?
Cheers
Trev