Welcome to the forum.
Has the system been checked for excessive voltage drops?
For example, if one connection is bad, such as for the contactor that enable / disables voltage to the system, with that bad connection consuming excessive voltage when under load, then less voltage is available for the windlass motor.
The system must be operating to locate the problem, such as removing the rode and operating the windlass motor.
Use a DC voltage multimeter and pointy probes, touch the conductor strands w/ one probe (lightly) to not improve / change the connection and touch the lug stud w/ the other. The measured voltage should be low, such as 0.2 VDC.
The other option is to remove all battery negative terminals, then disassemble, inspect, clean and properly tighten every connection, from the battery positive to the battery negative terminals.
