Even if you have an accurate phone or address, you may not be able to locate or make contact with your sample. Let's say you draw a sample of households in a small city. A household is not a respondent. Similar problems arise when you sample groups, agencies, or companies. Lists of various groups are extremely hard to keep up to date.
People move or change their names.
Even though they are on your sampling frame listing, you may not be able to get to them. And, it's possible they are not even on the list. Even if you are able to solve all of the other population and sampling problems, you still have to deal with the issue of response rates. Some members of your sample will simply refuse to respond. Others have the best of intentions, but can't seem to find the time to send in your questionnaire by the due date.