Board Thread:General Discussion/@comment-28260770-20160420195541/@comment-4453062-20160420201504

Belle explained this in the episode "The Price". She said that it did work, but ever since then, the darkness inside Rumple has put its guard up. She further explained that when a Dark One wants its power, it is no longer a curse. So it's a bit convuluted, but they did give a canon explanation.