What could cause a garden area to not let plants grow - but otherwise be healthy?

Many gardens contain either clay soil or sandy soil. Sandy soil consists of large particles that allow water and nutrients to drain away too rapidly to be of benefit to the root system. Clay soil, on the other hand, is made up of tiny layers that are so closely bound together that either the water cannot penetrate the hard surface, and so runs off, or it soaks in only to get trapped, thereby suffocating the roots of the plants.

Plant roots thrive in soil that has a balance of particles that hold on to enough moisture to keep roots from drying out while allowing excess water to drain away. Gardeners call this mixture loam. In such soil, air circulates freely, allowing microorganisms to go about their work of adding nutrients to the soil.

They ARE NOT HEALTHY if they are not growing. You are missing one of three things , the right amount of water applied at the right time when the plants need it or sunlight or fertile soil. Excuse me I forgot the fourth condition temperature, some plants are very sensitive to the temperature.

I'm thinking not enough sunlight.