Hi Alex,
It could be that the brushes supplying the armature windings are partially stuck or worn. You will have to remove and strip the motor to check.
It's not difficult to do this, but you must mark the steel motor body where it screws to the aluminium end parts. This is to ensure the brushes are in correct orientation with the magnet pole pieces in the motor body: if not it won't work properly when re-assembled (yes, I learned the hard way!).
I found the most common fault was damage to the driving end of the armature shaft and/or the bendix gear. Also the bendix can jam due to accumalation of dirt, or general lack of lubrication. Examine the bearing on the driving end of the armature, it's not really up to the job and if/when it fails can cause chipping of drive teeth.
Of course if the battery is lacking in charge, the motor will not spin quickly enough to make the bendix engage with the starter ring, but I assume you will have checked that out from the start.
We used to pull start our engines, leaving the motor as an emergency should Aidan stall on the track: they last a lot longer that way.
Hope this helps.
Dave.
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