If the motor has been operational for some time with no issues then all the sudden you are pushing plugs out and there is no major changes in the environment you are running it in then you have an issue that has come up on the motor like an air leak, water leak, etc... If you could give some more of your situation about how long the motor has been together, elevation, temperature, etc. there are many here who can probably get you lined up with your problem in a short time!