Exactly as Morfesto explained.
The valve spring is lighter than the main spring. When the marker is degassed the hammer just pushes the powertube back with little resistance. When gassed up the powertube doesnt give allowing the needed travel for the hammer to catch the bolt.
This also explaines why your valve wil "pop" when your gun is low on air and you attmept to cock it.
"I wish simply to be a decent person, yet I will always fear the retards" -Brian Hindt(geech) MCB Moderator Kidney Machine