First make sure the blades are not overly tight which could cause that problem.
In my experience repairing this type of equipment the problem usually is in the gearbox which located under the motor and will have a cover with six or so torx/Allen bolts. Remove that and inspect the gears for missing teeth.
It is fairly easy to disassemble but when replacing the gears ensure that you clean extremely well so as to not leave behind any broken metal parts in the grease
If the gearbox and teeth look fine, the next thing I would do is inspect the clutch, clutch drum and bearing ( if applicable). Let us know what you find.
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