User blog comment:IamJakuhoRaikoben/Chapter 496: Forward, Review./@comment-4449950-20160802062620/@comment-92.7.153.73-20160802171325

I think Gray and Lyon had trouble with Ur because, unlike the other Historias who were enemies (bar Simon), she was their adoptive mother and they had already witnessed her "death" once. We also know this weighs heavily on the minds of both Lyon and Gray, being vital to both their characters. Gray blamed himself for Ur's death and the very idea of "killing" her again would just be too much. The battles with the Historias are more mental than physical, requiring characters to get over the "mental block" in front of them and also their own pasts. As such, it makes perfect sense to me why both Lyon and Gray had the greatest struggle against the Historia (Ur) as it was a battle of mind and not power.