gd, what you describe if it has a Magneto Ignition is so often caused by a faulty magneto spring wind and trip impulse assembly.
It could have a weak or broken spring or the spring wind up and trip mechanism isn't functioning.
If you roll the flywheel over (subject to mag) every 180 degrees of rotation you should hear a definite loud SNAP not any faint thin hard to hear clunk lol
If (subject to mag) you were to remove a plug with it still connected but grounded safely away from the plug hole and turn the flywheel over every other time the mag spring winds and trips (if alls well) and SNAPS the plug should fire
EVEN IF THE MAG IS OKAY it could be be due to the start timing is incorrect. The mag should SNAP right at TDC (LHI mark on flywheel at 3 o clock), if too late its harder to start if too soon it can kick back.
If its a battery powered coil distributor ignition it could be a low voltage problem.
There ya go