For what it's worth, I have one that is intermittent but always starts easily.
After you've started if it's warm the light want stay on. Also, in addition to the clunk, watch the voltage gauge. On all of mine when the plugs kick in you see the drain on the batteries. When it's done, the voltage gauge will move to the right.
I had fits with one, switched out glow plugs and 4 different control boxes and it was something different every time. One light would flash, one it would blink, one stayed solid but still the glow plugs didn't work.
I still don't know what the wire went too 100% but I had a wire fried in the dash, got it fixed and had to put a second set of new glow plugs in and grounded and haven't had an issue since with that one.