Did you get a warning message? I can't recall the message I got but I know that the one and only time I did Coil I had one person who couldn't have any protective/healing spells cast on them. It was as if they were behind a barrier in a boss fight and I was outside.

I think we solved it by me completely running out of the sight, not just line-of-sight, of the other players and when I was back it was all good.

In short, sometimes in Coil things can become unresponsive to any magic abilities for no apparent reason.