DIAGNOSIS The milk wasn't pasteurized (heated to 180°F) before incubation. The "Yogurt" button has two stages: "Boil" for pasteurization and "Yogurt" for incubation. You likely skipped the first stage.
FIX For homemade yogurt, you must first pasteurize the milk. Pour milk into the inner pot. Press "Yogurt" until "Boil" displays. The pot will heat to 180°F and beep. Let it cool to 110°F, then add starter culture. Press "Yogurt" again to select "Yogurt" for the 8-hour incubation.
IF THAT DIDN'T WORK Use a standalone thermometer to verify temperatures. If the pot doesn't reach 180°F during "Boil," the heating element or thermostat may be failing. Contact support.
HEADS UP Using ultra-pasteurized milk can sometimes fail. It has less protein for the culture to bind. Use whole or 2% regular pasteurized milk for best results.