Why does my jg ak47 have low fps?

I just bought a jg ak47 at a swap meet, it was brand new in the box. when i got it home i charged it and shot it with .2g bbs. They immediately curved down after about 5 feet, i adjusted the hop up, but the bbs still came out with low fps and curved down. I then used .12g bbs and after adjusting tthe hop up, they curve down and look to be shooting about 190 fps. I spent $85 on this gun and want to know wats wrong. Thanks