I have been there, so many times lol.
Most recently, the group kept wiping because the DPS was dying faster than I could heal them. Since I'm in BH gear and have done all the HMs to death, that points to a problem with them, not my heals. They were not moving out of the giant circles on the ground and kept piling up on the boss instead of taking care of the adds. But of course, they started trying to "help" me heal them by giving advice. "Stop putting down your heal circle, it wastes mana." "Did you try keeping HOTs on us?" "Try shielding us" -- all of which I was already doing. Except for the heal circle part. Telling a corruption sorcerer not to use their heal circle is like telling them not to heal at all, lol.
All my attempts to tell them what to do were of course ignored so I finally said I needed to go, quit, and requeued. :P
Sorry that bit about not putting down the purple ring made me laugh out loud at my desk. Don't use your single most effective ability, which is incredibly force-efficient considering the healing it does on a melee group?
I did a LI HM once with my scoundrel healer. I had 3 people from some other guild and me. We wiped a couple of times at Dr. Lorrick, and I was repeatedly accused of not putting out enough heals. Hello! Almost all of the damage in the survival phase can be avoided. The damage is manageable in the burn phase if you take him down fast enough to avoid too many stacks of debuff. If I can't keep up with the damage you're taking when I'm geared in Rakata and I'm pure healer spec and I got us past LR-5 and Sav-Rak, it's not me!