I would also be frustrated by that, and I agree that DL needs to be better about regularly checking the quality of all of the little features they put on planes that people pay for and expect. On multiple trips I've had power plugs that were worn out so much that whatever I was plugging in would not actually stay in the outlet.

I'm assuming you know this, but it's important to use the "comment/complaint" link on the top of the delta.com homepage and write DL about what happened. They will almost certainly give you miles or a credit as compensation, unless the FA already did this for you on board (and I know that you would rather had the outlet working than have those miles).