Why Does the Pull Cord on My Gas Mower Slip?

I have a 22 inch Bolens pushmower I got at Lowe's last year. This year, the pull cord snapped and I had to replace it. While replacing the cord, I also took the blade off and sharpened it, and also changed the plug.

When I put the shroud back on and pulled the cord to start the mower, the flywheel only spins about half way around and stops. If I pull it easily, the cord will come all the way out and flywheel spins. When I pull hard to start it, it spins and stops. Nothing is blocking the blade, I do have the tank primed and the safety lever is squeezed.

Any ideas on why this happens?

By Doug Truter from Springfield, IL