User blog comment:Marco the Marine Admiral/Theory: Evilness Can Lead to Power-up for Dragon Slayers/@comment-4829046-20140628160016/@comment-7418318-20140628162031

To be fair, we didn't see more than short glimpses of the fights against the Dragons. With Sting and Rogue we only saw them before, between or after fights and never actually fighting.

So it's possible they did try to activate Dragon Force, but ran out of juice due to it comsuming a lot of magic power and having to revert out of it.

Laxus might have avoided going into it, to keep the collateral damage at a minimum and not accidentally hurt Wendy or Mirajane.

And with Cobra we only ever saw his opening punch, if I remember correctly, so we really can't tell if he went into this mode or not.