I may be misinterpreting the problem but it sounds a bit similar to a problem we had with ours a couple of years back.
Under load it would simply stop working but if I put the winch handle in and cranked then it would usually then give assistance as I manually cranked.
I removed it and pulled it apart only to find that water had got into the gearbox and over time had stripped most of the gear mechanism. The clue to the whole problem was a constant leak from the bottom of the windlass itself.
The windlass is a different make (Muir). They suggested I replace the whole thing but instead I simply replace the gearbox. In hindsight that was a mistake and I wish I had simply taken their advice.
So check for any leaks under the gearbox.
Apologies if I am right off the track.
“Life is a trick, and you get one chance to learn it.”
― Terry Pratchett, Nation