Have you tried checking the resistance of the injector coil?
It should match the resistance of the ones that work if you don't have the exact specifications. Typically 2 or more ohms but each system is different.
Excuse my lack of knowledge on this system but how are the injectors being powered?
I mean, do all of them have voltage going to them?
A typical system will have voltage....usually 12 volts.... going to the injector and the computer switches the ground. So, if the injector that's not firing has voltage going to it, then it's either faulty or the computer isn't grounding it either because the driver is bad or the wire from injector to ECM is open
If the injector is being powered, you can unplug the connector at the computer and see if that voltage is present going into the computer. If not, you have a bad wire but if there's voltage then it's likely the driver in the computer has failed.
One VERY important thing you need to be aware of if you decide to replace the computer, is that sometimes it's voltage spikes from a faulty injector coil that will destroy the injector driver in the computer. If that happens and you don't replace the injector then your new computer might also be damaged.
And, if something like that is happening then you need to consider that other injectors could be failing in the same way.
Sorry for the dark message but you need all the information you can get before spending the big amount of money a new ECM costs.
Good luck.