either really low compression or the injector is not squirting. injector could be mechanical such as debris preventing it from operating or electrical. if it's electrical it should pop a codeSo putting a new connector on did not fix the problem what would cause an injector balance rate to stay at 15?
15 is the max compensation the ECM can do. if it is a negative number then the injector is probably leaking or putting in more than the ecm wants. if it's positive then the ECM is not seeing that cylinder contribute enough and is trying to add more fuel. so, since your cylinder is maxed at 15 I'd look at why it isn't getting enough fuel or power out of that cylinder(s)