It's one of(or probably several of) a couple things:
  1. You're overhealing too much
  2. You're not using Shroud of Saints on cooldown
  3. You're not taking advantage of Freecure Procs
  4. You're using spells inefficiently (ie Cure I is more efficient than Cure II, Medica II than Medica I... etc)
  5. DPS/tank are taking too much avoidable damage, this includes when the healer/dps is taking aggro damage from lack of tanking
  6. DPS sucks and it's taking you too long to kill stuff

The first four are fixable, the last two you can't do much about other than yell at people.