I had the same issue intially even with my raid geared pally tank. The shamans in leveling gear and leftover S4 stuff didn't cut it.

I dual-specced one of the shamans to resto and replaced Flame Shock with Earth Shield and Chain Lightning with Riptide on the resto shaman's action bar.

The resto shammy doeesn't do that much damage, but the two HoTS together with 1x resto power healing totem and 3x normal healing totems are enough most of the time. If I should ever need more healing, I pop 4x lesser healing wave and that's it. =]