Facial structure has a lot to do with how hard or how easy it is to get the arrow under your eye. I guess I'm blessed, from an archery standpoint anyway, with having a narrow jaw and eyes that are set further apart than average. So I can be under my eye or on either side of under my eye, depending on where I anchor. Some people have a face that is wider at the jaw level than eye level, and I would imagine they have a harder time.
Of course, all this is predicated on keeping your head upright and unmoving as you draw and anchor, as is the current orthodoxy. In the recent posting of an old article by Jim Ploen, he recommends tilting the head so it is above the arrow after reaching full draw. Following my tendency to try everything different that I hear or read about, to the detriment of my overall archery proficiency, I'm sure, I tried this myself and found that I was more relaxed and my eye found it's position over the arrow as easily, or more easily, than I could position the arrow under my eye.