I also noticed that I wasn't getting my elbow all the way back when I anchored in the corner of my mouth.
In trying to correct this problem, I noticed a couple of other things:
If all you do is pull the arrow further back, you're not really solving the problem, and your elbow tends to stick out even when the arrow is hauled back quite a ways. You need to engage your back muscles, which will pull your elbow around into allignment with the arrow. It feels more like a rotary motion of my elbow than a straight back motion, and doesn't require as much of a change to get your elbow into allignment.
In Precision Archery by Ruis and Stevenson, they recommend using back tension as your anchor, rather than some point on your face. It doesn't mean that you abandon touching a particular point on your face, since that's still your rear sight. But it means that you focus on reaching a certain state of back tension each time you shoot, and don't release until you achieve that level of back tension. They point out that when you get tired, your head may droop forward, or your shoulders may slump, and if your only reference is a point on your face, you will start missing shots and not know why.
Terry Green, in the Tradgang DVD, recommends dragging the knuckle of your thumb across your face as you come to full draw. This assures that you have the same allignment of your bow with your face each time you shoot.
Making the three changes above has improved my shooting, although being aware of reaching a certain degree of back tension is subtle, as it's not something we have focused on most of our lives. But if you look in the mirror, and focus on the feeling in your back when your elbow comes into allignment, you will find that you can recognize the feeling when you're shooting. It only required me to move my point of contact back from the corner of my mouth to a point directly back of that on my jawbone, about 1" I would guess. A side benefit is that I pick up an inch in draw length, which brings me pretty close to a 28" draw length.