You might also want to look at the balance of your stats and think about the type of damage that you have to heal. I'm certainly no Shaman expert (my main is a Druid), but I've healed with a Shaman and done a bit of reading up on them. I know that the Shaman mastery works best when players are at low health, but if you are healing targets that remain at relatively high health for most of the fight you will probably get more healing out of crit. (I think haste is generally considered better than either mastery or crit for Shaman, but you might want to check that.)

Anyway, my point is that if you have a lot of mastery and your raiders aren't dropping very low in health before being healed then you might want to drop mastery in favor of crit. Or, if people are dropping to low health, try stacking more mastery so you can bomb bigger heals on them. Also check to see what stats the other Shaman in your guild are stacking and if you're doing something differently you might want to try their gearing strategy.

The blog Life in Group 5 had an excellent post comparing Shaman mastery and crit: http://lifeingroup5.com/?p=2258

After you've done a bit of research you might want to go back to your raid leader and let them know that you've studied your class, changed things around, and want a chance to show how you've improved. Your initiative and desire to improve your gameplay should impress them, and hopefully you'll get the chance to prove yourself again.