Be sure to follow the manual's directions on how to adjust your hop up.

Depending on your gun's velocity, you will need at least 30 yards of range to set your hop up. Try to achieve as flat a trajectory as possible.

Almost all Airsoft Electric Guns have very sensitive hop up adjustments. A minute movement of the adjustment wheel or arm can cause large variations in BB trajectory. 

Hop up mechanism can move and shift as the gun is being used. Expect to reset your hop up periodically.

Many factors can affect your gun's trajectory: cleanliness of the barrel, brand of BB, weight of BB, wind, humidity, and air density. Don't assume your gun will fire the same way anywhere, anytime, anywhen!