I AM thinking that in order to communicate via the nightmare …. several people have to be afflicted in order for the communication to occur. Snow’s nightmares might not have been the same as she had other worries…. THE CURSE being activated. Snow also did not have a night mare partner… She was the only one who had been put to sleep at the time.
Henry and Aurora were asleep in the same time frames. I also think that there is some kind of mystical attraction between FTL-EF characters and SB characters. (This theory is not quiet ready for discussion in depth… one day I will flush it out and post it.) I think that is why Henry and Aurora connect through the nightmare.