If an economy is not producing at full capacity, its resources are said to be (one word).

Underutilized.

When an economy is not producing at full capacity, it means that its available resources—such as labor, capital, and land—are not being utilized to their fullest potential. This underutilization can lead to decreased economic output and can result from various factors, including insufficient demand, economic downturns, or structural inefficiencies in the labor market. Such underused resources represent a significant opportunity cost, as they could otherwise contribute to increased production and overall economic growth. Addressing this issue often involves implementing policies that stimulate demand, enhance resource allocation, or invest in skills and training to move resources into more productive uses.