User blog comment:Omega natsu2/Chapter 357: The Nine Demon Gates, Predictions./@comment-5409550-20131020040405/@comment-7418318-20131023115455

Dragon force and dual elemental...

Maybe we didn't see Dragon Force on f-Rogue and Natsu in the GMG because having two elements somehow interfers with it and they'll only be able to go into DF again once they gained full control their new element.

That would explain why Natsu didn't enter DF despite eating Atlas Flame's flames (so awkward to write...) despite them surely being far more powerful than Jellal's flame of guilt.

And it would also explain why f-Rogue didn't enter DF despite having the ability to do so at will in the past.