Engine cranks but does not start

We have replaced the ECM, the coil, the rotor, the plugs. She is spraying fuel like she's supposed to. She's got a new fuel filter, new radiator, new tensioner pully (the old one broke), all within the last year, and her battery is relatively new and a heavy duty my husband had purchased for his F350 Diesel. The only other known issue is that the smog pump has cracked at the plastic nut where it connects to the pipe below. She does not leak any oil. Purrs like a kitten when she's running, and no known exhaust issues except that smog pump.

She cranks, but will not start; only occasionally tries to hit. The diagnostic tester says distributor, the coil, the plugs, the ignition module are all working. We are out of ideas. This truck is my only transportation to work and to haul hay and I desperately need to get her running again. Help is appreciated.

A/C is working but I rarely use it.