Limptwiglet
The standard Impulse is finely bablanced when firing.
If there is no ball in the breach then there may not be enough back pressure to ensure that the solenoid is fed enough gas to return the hammer (and therefore the bolt).
If it is doing this with a ball in the barrel - then you have way too much dwell - set the pressure on the reg to around 180 psi and test the gun, adjusting the dwell until it feels responsive and there is a little kick - the dwell is then good. Shoot over the chrono - adjust the pressure a little and then do the dwell again if it doesnt feel the same. Do this until it is right - then leave it alone - a Freak kit should mean that you won't have to adjust the pressure too much again.
OR - The Full Flow Valve - the solution to this;
The Full Flow Valve was developed in conjunction with the Rushers (hence we know what is going on with these cuties) and it is this bit of kit that will solve the problem, make it fire sweeter, and be more consistent over the chrono.
There are plenty of pretenders out there that go part of the way to solve the problem - the FFV goes the whole hog and is the best solution!!!
Good luck
Mark
PS ignore Famines advice (are you a Mag owner mate??) If it sticks forward - pull the bolt back with the pin. Mind you once you have the FFV - no more stick!!!