What's up with healers not healing?

[akaneakki]

Honestly, as a WHM main, I see this mostly happening with SCH co-healers during trials. p.

You have thin air, Saints and assize. I solo healed it on first try since healer dc. I dpsed and kept people alive, so you must be doing something terrible wrong like over healing. Whm shouldnt have mp issues.

[Driskus]

You have thin air, Saints and assize. I solo healed it on first try since healer dc. I dpsed and kept people alive, so you must be doing something terrible wrong like over healing. Whm shouldnt have mp issues.

Not saying that I run out every time. I don't have issues if people aren't dying constantly and the co-healer is at least contributing to some extent. It's more of a problem with running out when things are going very wrong, such as DPS constantly eating the floor from not dodging (you can't always get multiple raises off during Thin Air if you end up dodging multiple AoEs yourself), and occasionally with tanks who refuse to use tank stance & cooldowns (they need almost constant babysitting to keep them up. Sometimes Stormsplitter will still one-shot them, which usually results in more chaos). When SCHs are just sitting there with nearly full MP while people are either dead or about to die, something is horribly wrong with that SCH, since at that point they obviously should be trying to do SOMETHING as far as emergency healing instead of just sitting there continuing to DPS 100%.