Curve balls

I think this is a bug on the game, it happens to me a lot to.

Same here. Every now and then I also get a curve ball that just flies straight and miss the target completely.

If you are spinning counter-clockwise, the ball has to hit pokemon on the left side (left half of the screen) and clockwise, hit the right side. Absolutely nothing else matters. Source here http://pokemongohub.net/throw-curveballs-correctly