Many eligible people do not participate in the Food Stamp Program. Participation rates are especially low for eligible elderly Americans (.pdf), for example. These participation rates are strongly influenced by arcane program policies. A tangible policy is the length of the certification period -- the amount of time before a participant family must come in and prove its continued eligibility (see this USDA/ERS report and Nader's and my earlier article on that topic in the Journal of Human Resources). Less tangible policies affect the "climate" or "unfriendliness" of local food stamp offices. If barriers and hurdles are put in front of eligible applicants, they may become eligible nonparticipants. How is your state doing?
Here is the latest map (large .pdf), released this month by Mathematica Policy Research, Inc., a contractor for USDA's Food and Nutrition Service.