It's not at all surprising for you to fall behind Druids or Shammies in HPS. Your strength is in single target heals. Druids have HoTs that can heal people up before you even see their health bar move, and Shammies have the most efficient AoE heal in the game. The only way a Holy Pally can keep up is in situations where there is so much damage going on you get full use out of Beacon of Light. Even then you'll probably still be lower on the meters because of your role. It doesn't mean you're a bad healer.