You could have air in the fuel lines. Have you recently changed the fuel filter?
Check for bad fuel lines that may allow air into the system and fix this first. If you have air in the system it will have to be bled. To bleed the fuel system you need to purge the air from the system. There is typically a bleed screw on top of the filter housing to bleed the air out of the filter and lines leading to the fuel filter. Then you need to bleed the injector pump by cracking the injector lines on top of the injector and crank over the engine till you don't see air in the fuel that is coming out. Do all of the injector lines. Then try to start it up. If this does not solve your problem you may have a bad injector or an issue with the injector pump.