Take the buttplate off reach in there with a wire hook and pull it out then take out the spring it should have one,if not you can get one at the local hardware store that works. Clean out all the cosmoline and off the spring and cleaning kit reinstall buttplate insert cleaning kit and see if it comes out when the trap door is depressed.
The stock wood may have swollen and locked it in place. I had one do that before. Remove the buttplate and try to pull it out. If the wood is swollen, you might have to take a round wood file to it and widen the hole a little bit. Not too much, just enough that it can move freely.