Just a few off the top of my head.
1.Valve adjustment.
2.Low compression.
3.Weak spark.
4.Clogged/plugged air filter.
5.Dirty carb.
6.Bad deck bearings.
7.Bad deck pulleys.
8.Grass built up under the deck.
9.Deck is too low in the grass.
10.Tires are under inflated. Normal for this time of the year.
11.Water in the fuel system.
12.Vacuum leak/s.
13.Mouse nest in the air filter.
14.Mice chewing wires??
15.Old stale fuel from last year.
16.Plugged up slash carbon fouled spark plug/s.
17.Carb float too low or high.
18.Dead head gasket.