Disco 1 Starter motor problem

Hi im looking for a bit of advice on a weird starter motor problem.
The motor has been sitting in my garage for about six months and the one on the car is knackered so I decided to rebuild the spare one. When I benchtest it it's got plenty of power and the solenoid kicks the gear out with a lot of force.
When it's fitted to the car the motor just spinds but don't turn the engine.
Any ideas what would be at fault inside the starter motor

Make sure the earth leads are all clean and tight,might not be getting enough power on the vehicle,is the battery fully charged?.

X2 that, check all positive side connections and all earths right back to the battery, then if still not good try a "hotwire" jump from solenoid terminal on starter direct to battery positive.

Tried putting 2 jump leads on the starter with a wire from the solenoid to + and it's the same

clutch ion starter may be knackered,years ago it might have been a symptom of worn ring gear on the flywheel but ive rarely seen it to that extent

Have you put a jump lead from the battery negative to one of the starter mounting bolts? The earths go from battery to chassis, then chassis to bellhousing/engine bolts. I put an extra earth from battery straight to the engine.

Yer tried that gratch. Like James said looks like I need a new one. I like driving through deep water. Is there a water proof one I can buy