I was having trouble because I was trying to exactly emulate the way the demo video drew it (the one that plays when you approach your first tree). The demo shows a very large loop with medium-small tails. Even the shape that is displayed at the bottom of the screen when in the trees "soul" (or whatever the true technical term is for the mode you enter after you touch a tree with a drop of color) is not a very accurate image of what kind of shape is readily accepted. The game doesn't seem to recognize this method very well. But now I draw the gliff making a much smaller loop with much longer tails and it works every time (at least thus far).
For anyone else out there that is still having trouble with the supposed "easiest symbol in the game", there is a very good chart posted by Halfgild Wynac in the Steam forums that shows a number of accepted shapes, and an additional YouTubeb demonstrationon video to boot. The link:
http://forums.steampowered.com/forums/showthread.php?t=1087320