Without the model make and model number of your mower, I can only suggest that you unbolt the pull cord housing and see if the little teeth that engage with the notches on the inner face of the crankshaft pully are swinging out under centrifugal force when the cord is pulled. Clean and lubricate the mechanism if they don't. Replace with a new part if you find damage.
Above is general advice and may not apply to your particular mower.
Rupert, March 2020
