User blog comment:FairyRaven/Fairy Tail Chapter 414: Drops of Fire, Review and Commentary/@comment-5006572-20150105153905/@comment-7418318-20150105155743

I just had another thought.

Maybe the enchantment of Tenrou Island that kept FT members from dying and that strengthened their magic somehow interfered with the magic with which the Dragons hid themselves inside the Dragon Slayers and prevented them from coming out (or at least slowed them down so much that they couldn't make it in time).