could also be the battery is almost flat - enough to spin the starter a bit, but not fast enough to throw it in to engage
It is a pre-engaged starter so the solenoid engages and then turns the starter, this is done with two contacts on the back of the solenoid.
If the shaft is rusty (due to mud) then it won't engage and therefore won't spine.
If it is spinning it must be engaging so sounds like missing teeth.
Put the wheel brace on the front pulley and turn it a little (remove the wheel brace) and try again.
If it starts then it has teeth missing on the fly wheel.