Zero Turn Mower. I'm stumped.

Rule out a fuel delivery issue. Spray carb cleaner (good flammable stuff, of course) into the carb and see if that helps get enough power to engage the blades.

If it’s definitely getting fuel, I would assume it’s on the ignition side. My two cylinder Kawasaki had a coil crap out on one cylinder. Still felt smooth and started ok, but it was down on power. Replacing the one piece coil and wire assembly fixed.

Had a two stroke dirt bike with one cylinder do the same thing; down on power but started and ran smooth.