Check the obvious but I had an incident once where the injector pump in a boat that had been laid up got stuck by either a little rust or fuel gunk. At the time I had looked at rebuilding it but was advised by a Yanmar shop manager that rebuilding them was an iffy proposition and that he recommended just installing a new one. Before deciding to order a new one, I had immersed the pump in clean diesel fuel and started manipulating it manually and it finally released and seemed to operate smoothly. I installed back into the engine and it started and ran well for years to come. There was a lot of water in the fuel tank, more than would have allowed the engine to start and for that reason I suspect a little rust may have developed internally. I removed as much water as I could with the water separator and mixed in clean new fuel and once the engine started I then had the fuel polished.