User blog comment:Thewaterdragonslayer/Personal Reflections on the Differences Between 1st Generation Dragon Slayers and the Other Generations/@comment-112.198.82.250-20130208040018

Do you still remember Makarov's opinion about second generation dragon slayers? Right. They aren't real dragon slayers after all. Well that is according to what the anime said.

Moving on, it could be because of that infused dragon slaying lacrima infused on the second and third generations' body.

Though it may seem vague, I have a feeling that first gen dragon slayers have that unique ability to not use too much of that lacrima since having received a dragon slaying technique might have caused one to use too much of that space. Unlucky for third gen but who knows?

In the end it's a mystery until official explanation comes into place.