How about a weak battery, bad connections. You have to spin the engine fast enough to heat the air for proper combustion. It's winter now and batteries are at their lowest, oil is it's thickest, difference between ambient air and combustion temperature the widest to name a few.
Other thing is compression. If the rings are shot you will need a helper as I learned on the 3000 I own currently. I was new to diesels then and when it needed a shot of e go get it to crank on the dealer's lot in the spring, I should have RUN.
It's overhauled now and starts easily; oh and I have a new 5" starter, battery, and cables....stuff I bought before I came to the realization that it had shot rings.