He watched her try again, nodding as she fired the arrow. "Catiel, here." He said, lifting up his bow. "Watching carefully." 

He pulled back, filling the arrow with magic once more. "Watch carefully. You're filling the arrow with magic, true, but not fast enough, without enough force. Look at the speed of which the magic covers the arrows, the brightness of the arrow." He said, firing again, the magically enhanced arrow striking true. 

"You don't have to be exact to that speed or force, but it should be close to it. If you're too slow, the magic will start to weaken before you finish. If you do not use enough force, then the magic won't be able to last long enough."