There are a number of connectors in the wiring
harness from engine to panel, any one of which can contribute to a voltage drop. My panel has three sets of connections on series (transition between plug
types, apparently), plus an extension in the aft cabin
and the plugs at the engine. Check/clean the entire run.
In the end I added a cube relay at the starter instead of relying on the harness to deliver full voltage to the solenoid. If it clicks but does not crank, you have too much voltage drop for the solenoid to engage fully - needs at least 10-21 volts.