The Starter Rope is Hard to Pull or Seems Stuck:
A typical cause for this is that the engine flywheel brake (the bar you hold down on the handle that stops the engine when released), is engaged. Be sure to bring the bar down all the way to the handle, before pulling.
This can also happen if your mower blade is dragging in grass or clogged with grass clippings. Move your mower off your lawn to a hard surface. Then when the mower is turned off and the spark plug wire disengaged, clear the underside of the mower of excessive clippings, proceed to resume a safe mowing position, and give it another pull.