bullboss problem

Check the air cylinder while out of the gun.
The valve pin seal could have went bad.
There's a small o-ring on the valve pin and a inner valve seal that could be a possible issue.

Unscrew air cylinder and use a bit of soapy water and check for bubbles around the valve area.
I just make me up a clear glass of soapy water and gently submerge the valve into it to see if it starts bubbling.