It might be that your healing team has more healing power than necessary. While DPS can always go after bigger numbers, the healing for an encounter is necessarily capped by the amount of damage your raid takes. If your healing team's total output potential is higher, some healer(s)' output is not going to live up to potential. Sometimes, all the healers' output is lowered. Sometimes certain healers tend to get their healing in first and other healers see their healing drop disproportionately.

The first test of a healing team is whether bosses are dropping at the rate deemed reasonable for your raid team (based on time spent, dedication demanded, skill demanded, etc). If they are, the individual numbers of the healers is largely irrelevant (unless there's an extreme situation, like one healer doing 4k HPS).

If the healing doesn't seem sufficient in total, then it's time to dig a little deeper and try to really figure out where the discrepancy comes from. Obviously, it's a tough question, but the first step is to ensure you're being aggressive with your mana pool. Your mana pool plus your mana regeneration over the encounter is all "potential healing." If you wipe with mana left over, you left some potential on the table. If that's not an issue, then you need to start poring over spell choices and targets chosen to heal. That's where it becomes difficult. ;)