Has nothing to do with a lack of skill or anything tactical. There was a mini-documentary on it, how the quality of corners have gone down hill and why this is. The main reason was that the corners of most grounds are very congested and the grass changes to AstroTurf (The camera cables are under here, using grass would mean you rip up the pitch after every game when they take them away)
You'll notice here that there's barely any room and that there's effectively 3 ground types, as well as a slight dip in height. This tends to mean players are focused more on their footing and their feet, trying to avoid potential injuries than on the corner itself, especially since we use in-swinging corners where there is the least space
In contrast, here's the corner at Chelsea, a team that tends to always be in the top three corners-goals table. You'll see that there's a lot more space, the ground is even and you can barely notice the astro. As a result, no matter who takes the corners, we're going to struggle; we've been poor for the past 6-7 years. Christian isn't the issue. Just another very minor reason why the new stadium would be a bonus.