My coil group never has threat issues from tanks. I used to roll BRD for coil before taking the Tank spot when it opened, and our DPS give a 2sec wait, baically when the tank reaches mob and starts their first combo we start DPSing without holding back. If your giving them 10 seconds and still having issues your tanks are doing something VERY WRONG.

We have puged a few tanks when we were missing a member and some do it wrong and have alot of issues, we quickly end and find a good tank or dont waste our time. Tanking isn't that hard, follow a simple threat combo over and over mostly with a few other skills/combos passed in every few combos... if they cant do this correct, I dont know what to tell you to tell to them.