Healthy sourdough starter should be bubbly and active. The organisms in the sourdough culture are feeding off the flour and creating gases (bubbles). After feeding, bubbling action should be visible within 4 to 12 hours.
If a sourdough starter is not bubbly, it may require more frequent feedings. If feeding every 12 hours, increase to feeding every 8-10 hours, to make sure the culture is getting enough food.
Check the temperature in the culturing area. Sourdough likes a temperature between 70°F and 85°F, around the clock. To make adjustments in cold or warm weather, try the tips in these articles: