Sully doesn't respond as much to "Come" as he does "Come HERE!" When we say this, we point our finger directly to where we want him to come.
We taught him this by giving him a treat and LOTS of praise when he came. When he didn't come, we'd say "bad dog!" and try again. And when he did it right, treat and praise. He now comes for praise only, and he's really good about it. In fact, we don't even have to do both anymore. He will come to our finger pointing straight to the ground OR to our voices saying "Sully, come HERE!"
Best of luck, don't worry, it can be done!