Jim brings up a good point. There are 2 levers on the injector. One is on the top and is the cutoff lever that goes to the shutoff pull cable. This lever must be all the way forward. Run position for mine is all the way forward with the end of the lever almost touching the hand pump If something got up in there and bumped the cable support structure(above the clutch), this cable could be holding it to the rear and keeping the engine from running. I purposfully mis-adjust this cable when I go away to make the tractor difficult to start.
The other lever on the side is the speed selector/throttle and is connected to the hand lever and foot pedal.. For this lever, forward/down is idle and up and to the rear is full RPM. When cranking to check for fuel at the loosened hard lines, the hand throttle lever should be in the higher RPM area as this will move more fuel faster to the injectors and require less cranking. You should also be doing the cranking with the compression relief on to save starter wear.
As Greg mentioned, the fuel goes from tank, to bowl, to lift pump, to filter and then to the injector portion of the pump. The manual prime pump which is part of the lift pump portion of the injector assembly will only push fuel to the injector assembly, not thru it. I think there are also some mesh screens where some of these lines attach, so you may have to disassemble and inspect the fittings if you have fuel flow/delivery problems.
There are 3 bleed points to get air out of the system. These are also good places to check fuel delivery. The first should be on top of the fuel filter housing/boss usually located a few inches above the injector assembly. Loosen the bolt on top of the filter boss and pump the hand pump till fuel without air flows out the top of the filter, then retighten the bolt. The second and third bleeds are on the side of the injector assembly. One is right below where the hard line for #1 injector attaches, and the last one is right below #3. On mine one is a hex head with a standard screwdriver slot, and the other has a metal ring thru it to use as a handle so you can open it without tools.
You need to get all the air out of these points so the injector pump will pump only fuel. Once these are bled, open the hard lines at the injectors and crank the engine(short bursts and allow the starter to cool between crankings) with the throttle lever held at full till hopefully fuel comes out at those fittings. Once you have fuel at the injector fittings, retighten them and the engine should start. The injectors are fired by hydraulic pressure. Any air in the hard lines will, like a spring, absorb the energy from the pump and keep the injectors from spraying. A clogged screen at the tank outlet above the bowl, or on one of the soft line end fittings could cause the lift pump to pull a slight vacume. This could suck in air thru a loose fitting or pinhole and cause the injectors to stop firing.
Good Luck
If it is, then there is a chance that the fuel is not being delivered because of this situation. Ensure that the pump control is around mid position to ensure proper fuel flow and delivery. Once started, then adjust to what is a comfortable speed for your application.