MRL:
That happens when you don't fully pull back the cocking lever...FWB 300's have a mechanism to protect the user from hurting himself (amputating fingers) by the hammer sear breaking or the user pressing the trigger while the breech is open; it you partially open the breech and don't fully pull the lever all the way backwards, the breech will not close...If this is not the case, then you probably dropped a pellet inside the breech chamber or the pellet that you loaded fell off the chamber and it is stuck in there.
So: Open the breech and pull the lever all the way backwards, if you feel the compression of the spring then the gun is OK....Try closing the lever, if it doesn't then the problem is something stuck impeding the breech from closing...As said, check for a pellet or any other object in this area.
If you load the gun but then want to un-cock it: Pull the cocking lever all the way back until you feel the spring compression and while HOLDING the lever with one hand press the trigger and gently/slowly let the cocking lever go forward until no spring force is felt...Close the lever.
Regards,
AZ