Answer

There are a couple of things I can think of right off the bat, first the firing pin could be broken. If the firing pin is broken behind the firing pin flange then the mainspring is forcing the firing pin forward and the safety flange portion of the firing pin into its recess in the bolt. Normally when the bolt is cocked the firing pin is withdrawn via the cocking piece out of the safety flange recess and the bolt rotates so the safety flange does not line up with the recess in the bolt. So if the pin is broken as described above the bolt cannot rotate because the front half of the firing pin is in its recess preventing rotation. If this is the case the cocking piece should not have any spring tension on it. Another cause can be if the gun was fired with a hot load or a steady diet of warm loads over time, the lugs will set back into the receiver. Now to cock the gun the bolt has to move forward out of the recess formed and compress the cartridge case in the chamber and the bolt sleeve lock spring and the firing pin spring all at the same time. Yet another cause can be if the front of the cocking piece breaks off. Now there is nothing to rotate the cocking piece, and pull the firing pin back. The safety itself may be binding on the cocking piece. The trigger system may be out of adjustment so the cocking piece cannot move back because the sear is behind it and not being pushed down out of the way.