Apart from contouring, I think nailing eyeshadow application is the technique most people struggle to master. No matter how many tutorials you watch, for some reason, it just doesn't turn out the same way on yourself (and that one time you do nail it, the only place you're going is the supermarket).
But what exactly is it about eyeshadow that's so tricky? Is it the brushes you're using? Is it the lack of pigment in the powders? Are you not using an eye primer? Luckily, I've got a little experience in this area, so let's find out.