How hard is your hop unit on? Are you shooting single shots or a burst. What weight of bb are you using and whats the fps of the gun?
Hop: On to hard it is trying to spin the bb really hard, ie jamming it into the barell to try to get her to spin. This can provide inconsistinces in shot
Single shot/Burst: Single shot, you move. Only way to see wat your grouping is is to vice clamp the gun then zero it in. Also single shotting can mean the next shot is at a lower fps as the spring is partially compressed after the first shot, and thus you get dodgy results.
BB/fps. If you have a 350 fps rifle and are using .2's then they may go wayward as they are a bit light, using .25's tends to increase accuracy (they also move less in the wind).
Also what do you consider a bad grouping? I know that at 30m my gun hits a 12" target 18/20 shots in single and about the same on auto. The two odd ones are flyers, that spin up and away. At 40m I can hit a 20" head shoulders target about 16/20 times with a couple of flyers. Pretty accurate for airsoft.