This might happen when you try to regrow green onions in water. Plants need water, air, sunlight and nutrients to grow. When you regrow green onions in water, they have water, air, sunlight, and a small reserve of nutrients in the cutting. Those nutrients will eventually run out and the green onions will stop growing. When this happens, you have 2 options: 1. Plant the cuttings in soil 2. Use up the green onions and restart the process with a fresh batch of green onions from the store. Technically you could also grow green onions hydroponically which involves adding the nutrients directly to the water, but that is a lot more technical and not accessible to most people.