I wonder if there's some correlation between the amount of nitro used and the occasional start?
I think there is, the catalytic reaction is stronger with methanol than with nitro, and engines made for low nitro tend to have higher compression ratios. I suspect that this along with some built up varnish insulating the glow plug element can combine to create the rare unlit plug start phenomena.
Nitro isn't good at handling compression which makes me wonder if possibly it can act a bit like ether in model diesel fuel if the circumstances are exactly right.
Ether works in model diesel fuel because it has a very low ignition temperature. Not so with nitro, it detonates after it has ignited from compression of its rapid flame front, which is more rapid than that of methanol. Nitro is actually more difficult to ignite than methanol. With very rich mixtures it is also more difficult to ignite than gasoline which is why the AA fuel dragsters have such large magnetoes, in fact the electronic magneto gave a huge power boost to those engines because it allowed them to run even richer mixtures of nitro.