Don't know if you have a MG or MK motor, but as far as wiring goes, MK requires between 10ga to 4ga wiring and either a 50 or 60amp circuit breaker for all their motors 40# thru 101#. The variant is dependent on which particular motor you have and how long of a wire run you have. The only model MK sells that could get by with wire as small as 12ga is their 30#/12v unit, and then only if the wire run is 10 feet or less. I would start with verifying the wire & breaker are proper size for application. After that as it's just one battery feed having a new issue, I'd double check all wire connections for decay or corrosion, check that a wire route didn't get damaged by a pinch or kink. I for sure would not put in a larger then specified fuse. That would possibly lead to an on board fire or damage to expensive gear/equipment.
I do have a pdf file MK sent to me before I purchased my Terrova last summer & was wiring my new boat for it. I'd be happy to email it to whomever would like it for reference.