User blog comment:Marco the Marine Admiral/Why don't they activate Dragon Force?/@comment-7418318-20130719083852

We only saw Sting and Rogue after they were already beaten.

It's possible that they used Dragon Force, but were still unable to defeat the Dragons and sooner or later they just ran out of endurance and couldn't keep it up any longer.

And considering how F-Rogue was easily able to deal with Natsu without DF untill he was hit by that surprise finisher, it's no wonder he didn't bother to do it.

No idea about Natsu though.