User blog comment:FairyRaven/Fairy Tail Chapter 391: Gray vs. Silver, Review and Commentary/@comment-33019320-20140704095638/@comment-24800928-20140704161301

Why Gray's ice may not work could be because of two things

1) Gray is not a dragon slayer, so his ice wouldn't have any effect

2)devil slayer magic has a slightly different property.

pretty sure its the first one