There's been a lot of talk of this lately (and, quite frankly, since Cata brought in the new healing model), but shaman are tending to not have the same output as other healing specs. Whether it's because of skill or because people aren't playing them as much as other classes because they believe them to be suffering, or simply a case of the class being weaker is unclear.

That said, Jaba pointed out some very good places to start looking at your healing. Not knowing what your UI/healing setup is like, it's hard to say whether that could help, but it might do to look it over for yourself and see what you can cut out/add/change to improve performance.

However, you say that the other shaman is (are? didn't mention how many other shaman) using the same heals you are but simply getting higher numbers, and to me this sounds like a gear issue where your heals aren't hitting for as much. Or it could just be that the other healers in the raid are keeping everyone mostly topped off and you just happen to be using the same spells. It's hard to say, but I encourage you to keep at it :) Practice makes perfect, and that goes for healing, too. If you like it, you'll stick to it and the rest will come. Hope I helped.