Snowball game

It is playable, but collision detection is behaving very oddly. It does not seem to register collisions when they should happen, but says there was a collision at seemingly random times. The code looks like:

if ((o.vPos + (0.5 * o.getHeight()) + (0.5 * snowball.diameter) > 500) && (o.vPos < 500)){

if (((Math.abs(o.hPos) - (0.5 * o.getWidth())) < (0.5 * snowball.diameter)) &&

(Math.abs(o.hPos) - (0.5 * o.getWidth()) > (-0.5 * snowball.diameter))){

//do stuff

o is an obstacle. vPos and hPos stand for vertical and horizontal position. The snowball is located at 500 on the y-axis, and is at what functions as 0 on the x-axis. (in terms of pixels, this is actually 300)