On US and UA birds, empower jacks have a lamp by the socket. If the light is on, and green, empower is on. If it is off or red, empower is no working. I haven't flown DL in 6 years so I don't have any direct experience there.

Several months ago, I bought a Targus empower adapter, and had mixed luck using it. Some planes it worked, some not. By work I mean on some planes it charged my laptop, and some it didn't. In all cases, the blue light of the Targus adapter came on. Finally, on a flight sitting next to somone with an iGO, I noticed he was fine, and my Targus was not working. I borrowed his adapter and my laptop charged just fine in my empower jack with his iGO.

Conclusion: my targus was no good. Solution: buy an iGO. Resolution: bought an iGO and been very happy ever since.

Anyone wanna buy a barely used Targus empower adapter?

So I would look at #1 or a variation thereof as being your problem. The adapter might well be working to its design, and might be working to the
empower specfications. But it might not work with all empower sources, at least not when powering/charging your device. And that's what matters the most.

I look at it this way: iGO's whole business model depends on having products charge devices on planes. Everyone else is into that business as a sideline and it shows.