A friend and co-worker last week dealt with a 4WD tractor that was hard to start. The customer said he had to hand-pump the fuel primer to get it to start, which often suggests fuel is leaking back and draining the fuel injection system when the engine isn't running.
My co-worker did all the usual tests to diagnose fuel leak-back, and replaced various fuel lines, filter housings and other components that often cause that problem. He finally figured out that the tractor had to be hand-primed to get it to start, then it would start fine all day, even all week, as long as the engine wasn't pulled hard. But if the engine was pulled hard then shut off, it would take 15 minutes of hand-priming to get it to start.
He finally used a diagnostic procedure that included replacing one of the diesel fuel return lines with a clear plastic hose. At both idle and full, no-load throttle, the fuel in the line looked good, But if he pulled that engine HARD on the dyno, bubbles appeared, and the harder he pulled it, the more bubbles showed up in the return line.
His conclusion? The diesel fuel injectors and especially their seats/seals were worn and allowing exhaust gases to force their way into the injector, where they were introduced into the return line, which recirculates through the injection pump . When the tractor was shut off with bubbles in the fuel line, those bubbles would consolidate into big air pockets when the tractor sat for a couple hours or overnight. And air pockets in a fuel injection line are nearly a guarantee a diesel engine won't start.
It's not going to be cheap to replace all the fuel injectors, but the cost of spending all the time diagnosing and dynoing that tractor is going to be a big part of that repair bill. Unfortunately, sometimes it costs as much to figure out a problem as it does to actually fix it.