Certain voters, notably younger ones, are notorious for telling pollsters they plan to vote - when history shows they don't. Timing is another issue. Some pollsters poll on weekends. Others shun it. They fear they're less likely to reach married voters with children, skewing the results.
People are harder to reach and less likely to answer telephone surveys.
That makes it more difficult and costly for pollsters to achieve a representative sample. Thanks to the Electoral College, the real contest is for states, not the popular vote. But because state polls are done less frequently, those electoral maps on TV and in newspapers are sometimes outdated. There have been only a handful of public polls in Wisconsin since early summer. What's more, trends in the national polling don't always translate into changes in the states.