At a time when food prices are soaring, a growing number of Americans are struggling financially and local social service agencies are seeing record numbers of applicants, advocates are concerned that the purchasing power of food stamps has shrunk since 1996, when Congress recalculated benefit levels. The result slowed the value of food stamps relative to inflation. If benefits had kept pace with inflation over 12 years, a family with one working parent and two children would be receiving an additional $37 a month, according to the Center on Budget and Policy Priorities, a Washington-based think tank.