From what I hear, you don't get the dragon's soul if you've killed it once already. I killed the dragon at Bonestrewn Crest again last night and it didn't even decay, let alone give me a soul. I'm quite content as long as I get bones and scales out of the exchange.

I already killed Krosis again. Apparently I saved right after killing the dragon, but don't remember doing so. Checked the chest nearby and there was a staff of ice storm in it so I spammed the crap out of him with it.