Board Thread:General Discussion/@comment-25926288-20161205015930/@comment-25733960-20170109203150

She wasn't able to do it by the end of season 4, because she could have do it instead of killing Cruella

And then she did it in Camelot, a few after she became the Dark One. She froze Snow and Charming.

So yeah, we can assume it's a consequence of being possesed by the dark one curse