It could be some thing as simple as a defective injector. I'd check the injector to see if signal is flowing first.

 

Pull the plug off the injector and do an ohm test...can't member off the top of my head what it is supposed to be, but it shouldn't be infinity and it shouldn't be zero. Also check the plug to make sure its not too badly corroded and check to make sure that you have power on only one side of the connector. It is entirely possible that you have a bad injector. The place my shop buys ingectors through won't even rebuild the Z injectors due to too high of failure rate. Also, were the injectors replaced initially trying to cure a cylinder miss...perhaps on that same cylinder. If so, have that cylinders compression and leak down checked.