Rabies be right.
(howdy dude)
If you have an expension chamber use it. It will prevent liquid CO2 getting into the marker... and thats a good thing.
Have you had this marker since it was new?
Just checking for stretched springs or something.
Sounds like the "blowback" bit of the firing process isnt getting enough gas.
As rabies said, make sure the hammer has an O-ring on it, at the same time, make sure the hammer moves freely (ie the oring isnt making it stick)
Once youve done that.
Wind the velocity adjuster ALL the way IN!
fire the marker, and unwind it, "hopefully" the marker will start to cycle properly, and then you can chronograph it to a reasonable setting.
Hope it helps.