Either a wire or a connection is bad, or the starter itself has an issue that needs looking at.
You can wire in a 12v light bulb from the battery
terminal to the starter terminal and if the wire or a connection is bad it will burn brightly. You can do both positive and negative terminal. If the start solenoid is on the wire and not the starter you can test in 3 steps- batt to solenoid, then across the solenoid, then solenoid to starter. Make sure batt voltage is acceptable .
I'm at a Thanksgiving/Birthday party right now so will have to write more later.