Among the 7 dragon slayers, 4 could activate DF by will (Edit: Laxus' and Cobra's DF are considered fake now).
The last time Laxus activated (fake) DF is in the FF arc. He hasn't activated it ever since.
The last time Cobra activated (fake) DF is in the OS arc. He hasn't activated it ever since.
The last time sting and Rogue activated DF is in the 5th day of GMG arc. They haven't activated it ever since.
A regular pattern. All those 4 dragon slayers can only activate DF (fake or real) by will when they were antagonists. None of them have shown that feat again when they act as protagonists.
I often wonder why none of those dragon slayers activate DF when facing the real dragons. Now it can be explained.
What do you think?