Walk Behind Lawn Mowers

I have a self propelled walk behind mower. Not sure of the model at the time of posting.
Last summer as I finished mowing for the last time my blade fell off the lawn mower. I noticed the adapter was worn all the teeth were gone that the blade fits on. So I ordered a new one online. When I received the new part I installed it. I did take care to be sure the key was lined up. I put on the blade and tightened it down securely. I attempted to start my mower and pulled the starter rope about 3 times and heard the sound of the blade hitting the ground. The adapter teeth were again stripped off and so thinking I did something wrong I took the mower to a independant shop. The owner suggested I had purchased the wrong adapter and installed another one and put my blade back on. He started the mower and let it run for a minute. I then paid him the 75 bucks and took my mower home. It set in my shed until yesterday. I took it out and attempted to start it and within 3-4 pulls on the rope the blade had stripped the adapter and was loose.
Sometimes when I pull the rope the rope will jerk back or act like its caught on something.
What can be causing the blade to continue to fall off and strip the adapters?