Nobody mentioned the flywheel key.

Ensure that your blade is tightly attached, as this can shear the key. So can hitting a rock. The engine will usually continue to run with a sheared key, but not as well as before. Once you shut it down, it will probably be impossible to start.

You may be able to determine if the flywheel is in the correct place by removing the spark plug and feeling for the compression stroke. The magnet should pass the magneto-igniter at about TDC. Of course, the only way to really be sure is to remove the flywheel nut and flywheel and take a look.