My HW40 has dramtically lost power

Although their is some resistance as I close the gun (probably around a third of what there used to be), now there isn't enough power to move the pellet - there is just a faint sound of escaping air. It went from firing normally to marked decrease in resistance when closing/decrease in power to not even moving a pellet within the space of 4 shots.

Any thoughts on what might be the cause gratefully received

ATB

Jon