Like someone said, it's tough to give a definitive answer unless you're privy to Google's 'secrets'.
What I can tell you though is, there's such a thing as 'Google dance'. Your keyword might bounce around a bit before it settles and starts to rank once again. My best performing post did exactly that. How long? I don't have the answer.
In the meantime, keep building links: Blog comments, social shares e.t.c.