"Injector warning" should be renamed "injector, but more likely a dodgy connection in the plug" warning! I assume your ECU is actually giving a "dodgy injector" fault code? If so, read on.......
Injectors don't tend to "half work". Normally they either go or they don't. I would run a check on the wiring loom 'tween the ECU and the squirters, and pay close attention to the connectors. the fact you have managed a few hours running sounds like one or more might be shaking loose. (by disconnecting & reconnecting them when they were flushed has probably remade a dodgy connection in the plug(s), even just temporarily.
I don't know what kind of plugs (electrical on injectors, not spark!) the Opti uses, but if you can get inside and squeze the two halves of the electrical connection inside together a bit so it grips the pins on the injector better, it might make a more reliable connection. Then squirt some WD40 (or whatever your local brand of "save the universe" oil is
) to expel any dampness, and hopefully that should get you on the move again.