Sounds like it could be an issue with the poppet seal.
To check the poppet is seated properly, degas completely and remove the bolt guide from the marker, then remove the velocity adjuster by unscrewing it all the way anti-clockwise.
Take care as there will be a small spring in the back of the poppet body. As you've removed the bolt guide from the gat you can use an allen key to poke the poppet body out through the back of the bolt guide.
Check the poppet seal (white plastic) if its damaged and you have a spare, replace it. If its not obviously damaged, give it a good clean and apply a small amount of lube to the body and the seal itself.
At this stage it would be a good idea to wipe all the old lube off the internals and apply fresh. Personally I use Monkey Poo, but any Dow 33 based (sl33k/dye/gun sav etc) lube will be fine.
Re-insert the poppet into the bolt guide, adding the spring and reseat the velocity adjuster and turn clockwise until it sits snug. To get to a ball park 250+ fps turn the velocity adjuster 1.5 turns anti-clockwise and you can adjust using either the back cap or ASA to fine tune.
