Why is it difficult to include a proportionally representative cross section of society in the process of polling?

Question

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In order to include a proportionally representative cross section of society in the process of polling, pollsters must select a sample size that accurately reflects the demographics of the population. If they query too many or too few of various groups, they lose proportionality. This can be difficult to achieve, especially if certain segments of the population are hard to reach or unwilling to participate in the poll.

Why Is It Difficult to Include a Proportionally Representative Cross