Engine won't crank if recently cranked

[Ryban]

I'm pretty sure this is a battery or weak connection issue, but wanted to get some feedback to help me debug this week.

My Vetus M4.14 will always start, but has a few concerning quirks:

1. If the engine hasn't been recently run, I'll use the glow plugs for 10-15 seconds, but have to wait an equal amount of time before I can start cranking the engine, or I'll just hear a click of the solenoid, but no motion from the starter motor.

2. If I have been cranking to say, bleed the fuel lines (self-bleeding), for a short duration, I need to wait a good 20-30 seconds before I can start cranking again, or again, I just get click of the solenoid.

I suspect this is because whatever voltage the starter motor is seeing, it's below some threshold and it's not getting the power it needs.

I have two battery banks, one made up of 4 x 6V batteries and the other 2 x 6V batteries. They can be isolated, but I've experienced these issues with both banks combined. I'm not sure if I've actually isolated them and troubleshooted, because even when they are isolated by the main switch, there is a switch in the charging circuit that will combine them when they're being charged (which has been most of these times).

It could also be corrosion in the wires or on the terminals, so I'll check and clean everything when I'm back on the boat. I'll also do some voltage drop checks to see if I can narrow it down.

If it's not any of those things though, where else should i look?