Not tried on Delta, but I've had bad luck getting power outlets to work on various United planes; often it seemed like the issue was a loose/intermittent connection to the plug, with the outlet cutting off after a bit. Following a suggestion from FT, I tried using one of the European 2-pin adapters on my laptop's plug, and had much better luck. The round pins are longer and thicker and seem to make better contact. Worth a try...