I would have thought it was just a tap of square. as far as i heard, the power bar line thing just equates to the height of the shot and not the power. but i'm not sure.
You could try different timings of the shot, ie try releasing shoot when the ball is either close or far away to the players feet.
Also try, sprinting using the double tap R1. Then try using minimal power bar when your player comes to meet the ball again. if you know what i mean.