Elwyrr actually offers to help him with that glamour, explaining that it's a lot more difficult to achieve perfect invisibility, since it's very hard to hold the image of oneself being completely invisible. Especially for stronger personalities, as it requires not only holding an image of oneself physically invisible, but mentally, as well, difficult for even those who are truly skilled to master. Of course, a simple cover is usually good enough for most combat situations, forcing the enemy to waste effort on trying to pierce the glamour, and allowing for a moment of confusion.