IMO, many Trump supporters either refuse to participate in polls or lie to pollsters. They do this because they are afraid to admit they support Trump. Many Trump supporters have been victims of harassment and violent attacks. The result is that even honest polls fail to show the true level of support Trump has. On Election Day/Week, these people will vote for Trump. The pollsters and the media will be in shock at the results, just as they were with the Brexit vote.
And this is in part, due to the media not having great knowledge or insight into “middle America” or “flyover country”.
These liberal media hacks, who spend their lives in the New York/Washington axis, just simply have no clue about how real people, in places away from liberal hubs, view issues and candidates.