Since it worked ok before this sounds like corrosion
. Check all wire connectors from battery
to the whinch as well as to the point where the instrument bus is powered.
Alternatively the batteries may be toast and can't handle the massive loads anymore.
To limit you hjnting efforts:
Have you checked the voltage at the battery
when the whinch is running under load?
Have you checked the voltage on the instrument bus?
At least my raymarine
instrument has the possibility to view the voltage on the bus.
Or buy a boat where one can raise the main by hand