score improvement does not come from taking a good course. You will need lots of practice and sacrifice on your own part (less social outings, less tv.. etc) to nail the problems that are a bothering and draggging your score down.
A gmat course can only help you with getting your basics up to scratch, teach you some strategies, and help you perhaps familiarise with different questions and show you how you should solve them. That's so much they can do. The rest is up to you