The most common reason for this is either a hydro lock caused by the carb float valve not cutting off the fuel which fills up a cylinder,
or
The decompression not working where fitted, usually caused by excessive valve lash.
It would help a lot if you tell us the make & model number of the engine fitted.
Remove the plugs and turn the engine by hand, ignition off.
If you smell or see fuel then you have the carb problem
If it turns over easily and you do not smell or see fuel, replace the plugs.
If it is near impossible to turn the mower 2 full revolutions then you have a decompression problem.