Most likely, you simply need a new extractor spring. You can find them for less than $10 shipped. The extractor spring is the little silver or copper colored spring that you see on the bolt when you look into the ejection port. When they get weak, you start experiencing FTEs like you describe.
It could also be a bolt gap problem, but most likely just a weak extractor spring. You can try to remove it and bend it back into shape.
To remove, use a small punch, or a dental pic, or anything that will grab the rounded part of the spring. Rotate the spring 180 degress, and slide it out. Bend it and reinsert it.
To install, line up the spring the way it should go, and push it into the bolt. Sometimes it helps t roll teh bolt forward on something hard to get the spring back into the slot where it belongs.
HTH