Making yogurt at home gives you the freedom to experiment with flavors, textures and additives to find the exact combination that you like best. However, like any fermented food, homemade yogurt can sometimes fail. If your yogurt fails to thicken, the most likely cause is lack of fermentation.
Tweak Heat and Time
Fermentation, which causes yogurt to thicken, happens best in a warm environment. If your kitchen got a bit cool while the yogurt sat, fermentation may have slowed down. It hasn't stopped, but you may need to give it more time to thicken. Place the yogurt in a warm location to speed up the process. Your oven is a good place to let the yogurt sit overnight. The pilot light will add just enough heat to keep fermentation going at a reasonable pace. Keep in mind that the longer you let the yogurt ferment, the tangier the flavor will be. If you've let the yogurt ferment and it is still thinner than you'd like, put it in the refrigerator. As the yogurt cools, it will thicken.
If you are happy with the level of fermentation of your yogurt but would like the texture to be thicker, strain it through several layers of cheesecloth. This technique will give you a thick Greek-style yogurt. You can set up this method by placing the cheesecloth on top of a large bowl and securing it around the bowl's edges with a strong rubber band. Loosen the cheesecloth if needed and scoop yogurt into the cloth, letting it drain in the refrigerator for at least an hour or until it has achieved the consistency you desire.
Add a Thickener
Thickeners, such as nonfat milk powder and powdered gelatin, will increase the viscosity of your yogurt. Using thickeners works best for yogurt that will become part of another dish, such as a cooked sauce. The added thickeners will not be noticeable after cooking but will stand out if you eat the yogurt on its own. If you want to turn your yogurt into a dessert and don't mind a slight tang to the final product, combining thin yogurt with pudding mix can yield a rich, creamy treat.
As a last resort, you can restart fermentation if your yogurt completely failed to thicken. Reheat the yogurt, add another dose of starter, and let it sit in a warm place overnight. If you decide to try this method, consider starting a new batch of yogurt with fresh milk that has not been ultra-pasteurized, in case your choice of milk was what prevented your first batch of yogurt from thickening.