Is this a new (to you) rig? I mean, has this ever worked? If it worked previously, what did you change between when it last worked and when it stopped?
You say you've turned the velocity up; how much? Did you turn it until it started venting out the back? If not, go back and try again.
If possible, try using another air source just to be sure it's not an issue with your current tank/ASA. I've seen where some ASA's fail to fully depress the pin on the tank allowing enough air in to the valve to charge it, but not enough to fire it.
You say you've turned the velocity up; how much? Did you turn it until it started venting out the back? If not, go back and try again.
If possible, try using another air source just to be sure it's not an issue with your current tank/ASA. I've seen where some ASA's fail to fully depress the pin on the tank allowing enough air in to the valve to charge it, but not enough to fire it.


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