One of the most common caused for a "failure to eject" is a worn or broken ejector spring.  Springs are considered "wear" items and have a limited service life. 

AR15 Lower Parts - Selector and Ejector Spring

• Manufactured to milspec dimensions
• For use on M16, AR15, and AR10, selectors
• For use on M16 and AR15 ejectors
• 100% USA Made

One of the most common caused for a "failure to eject" is a worn or broken ejector spring.  Springs are considered "wear" items and have a limited service life. 

AR15 Lower Parts - Selector and Ejector Spring

• Manufactured to milspec dimensions
• For use on M16, AR15, and AR10, selectors
• For use on M16 and AR15 ejectors
• 100% USA Made