Line tightness
 

With the fuel jumper lines from the rail to the injector you need to tighten the he'll out of them to get them to seal. They are only good to be loosned twice.(gm says one time use only) I've done mine twice and now there is fuel in my oil. I have them over tightened and the still leak.

I had the return lines too lose on the driver side. The specs for the fuel supply lines on the rail and injector is 37 ft lbs. if they are clean they should seat properly. I had to bead blast mine and use a crow foot wrench attached to my torque wrench. That does create some deviation on the reading of the wrench. ( The calculations should come with the wrench.). I have heard you can also twist the injector when tightening and that creates a gap on the return causing a leak. Good luck!