Iowa 2016 had a lot of elements that you might have expected to produce a big swing in national polls. It was a decent-sized upset for one thing, relative to surveys in Iowa that showed Trump ahead of Cruz. But it didn't move the needle all that much.
There were some complicated reasons for this. Party elites didn't love Cruz. Rubio picked up some of the surge that "should" have gone to Cruz, then blew it in the debate. And Trump may have created some ambiguity in the outcome by accusing Cruz of fraud.
But there's not just uncertainty in the outcome for Iowa; there's also uncertainty in whether the bounce Iowa produces will be large, small, or (unlikely) nonexistent.
