A myth concocted by the press.
National polls showed Hillary beating Trump by about 3%, the actual vote was well within the margin of error, less than a percent off.
There was not enough polling in the MidWest in 2016, nobody was paying for it. Whose fault was that? Polling is expensive, it has to be paid for.
The whole myth turns on ONE poll of Wisconsin that was SLIGHTLY outside the stated margin of error. That’s it.