This happened to me back in the day when I used my UTG m324 (generation three). The hop up would be all set, bbs shooting on a flat projectory, and all was good. However, when I attached the scope, and looked through it (to zero it in), the shots would appear to go up, as if the hop up was set too high. Simply had to adjust the scope (turn notch to left--meaning "down") and took another shot from the rifle. If it was still appearing high, I would simply turn the notch again.
Not sure if this is the case for you though. Did you try to adjust the scope yet?
Not sure if this is the case for you though. Did you try to adjust the scope yet?