People taking part might not be a proper cross-section of the whole population. For example if a survey sample is skewed towards groups who are more Labour than the rest of the population then it will exaggerate Labour's support. People may give the answer that they think is socially right rather than what they truly believe. So some may say that they want to pay more tax to fund public services while privately feeling that they prefer tax cuts. Similarly some supporters of a party that is enduring criticism in the newspapers may feel shy of admitting their allegiance.
Polls can only give an imprecise view of the past rather than accurately predict the future.
Voters' decisions are often made because of emotional or instinctive feelings. Polls are much better are trying to measure things that are precise.