User blog comment:IamJakuhoRaikoben/Chapter 411: Quid Pro Quo, Review./@comment-24536822-20141202022006/@comment-9595671-20141202123846

Considering he was already near exhausted when he activated it I think it would have been hard to even, plus there's Sting and Rogue's DF we have to consider.

It seems more likely to me that it had a time limit, before he needed a special magic to aid him just to transform in the first place. I think now he's starting to slowly learn how to use it freely like our fellow Sabertooth Dragons.

Plus remember how Natsu said Sting and Rogue's Dragon Forces were half-baked? I think while those Lacrima let them use it freely, since its an ability that a DS can eventually learn their DFs didn't fully develop as a result and aren't as strong as they could have been.