The problem with polling Iowa is that a caucus is not a primary. With a primary, voters go to a polling booth, click the name of the candidate who they support, and leave. With a caucus, often not even in the same location as where voters go to vote in primaries, there is an hour commitment, the voter must be in the room by seven o’clock in the evening, and they must show their support rather publicly.
People who show up for primaries may not know the caucus location, they may not realize they have to stay an hour, and they may think they can show up late. Likewise, we know historic turnout models for caucuses and many of the late polls with a huge Trump lead have turnout models that far exceed even the most historic high turnouts of caucuses past.