You probably overheal a lot.

You probably aren't maintaining uptime on your regen.

You probably aren't using your cooldowns correctly.

You probably aren't using shroud correctly.

You probably aren't reading your other healer correctly.

You probably aren't reading your tanks correctly.

Your tanks are probably not using cooldowns correctly.

Your DPS are probably lacklustre, extending the durations of fights and taking a toll on your mana

Your DPS are probably getting hit by avoidable damage.

Any number of these options can mean you are run out of mana. Scholar AoE healing being superior to WHM AoE healing is a myth, they each are better in different scenarios.