I tried searching in the threads about the new Delta website, didn't find it, but if it was mentioned before, sorry:

Does anyone have issues with selecting EC seats on the Delta website? This has happened to me on 2 (out of 2) itineraries week (both domestic and intl). Go in, EC wide open, select seat, and then get a pop-up "passenger not eligible for Economy comfort". Not even an option to buy - just "not eligible".

Logged in, itins are all ticketed properly, it even says on the right hand side of the seat map that little ad for EC that my cost is "FREE".

Oddly, using the iphone app, no issues changing to EC seats and these changes then stick on the website as well.