It's because when the Dusks stole the photos, they sort of stole the data for the virtual people to say "photo." Roxas was probably affected by this in some way so that he couldn't say it either. My guess is that since the photos had him in it, along with some other virtual person, the virtual person losing the word could've caused him to lose the word in some sort of way too (maybe coming in contact with them, who knows!). Well, there's your first answer ^^;