I'd say in reference to healers not being able to heal the party is largely due to either inexperienced players, you aren't pulling aggro fast enough, the pulls are too big for the healer, or they just aren't used to healing the bigger hits.
You can also help this a ton by making sure you are rotating cooldowns correctly and stunning hits that you can. Stufff hits a lot harder this expansion and Astro and Sch are having slight issues keeping up. Just do your best to mitigate what you can and make sure if you have party members that can help mitigate stuff like dancer's shield samba that they are using it.