The usual one is the timing has slipped again…..
If so, remove the flywheel completely, and clean the internal taper of the flywheel and the external taper of the crankshaft, ideally with brake or carburretor cleaner, but clean petrol will do. Also ensure that the tapers are undamaged. Slight burrs can be dressed out with a fine file or emery stone.
Replace the flywheel and time the engine correctly do not apply any lubricant of any sort to the taper. Then, put the spanner back on the flywheel nut, and hit it with a hammer to tighten it up fully.
Villiers originally supplied a “Hammer Tight” spanner especially for this purpose.