DIAGNOSIS: That's actually by design, not a fault. The 'Keep Warm' function on this model has a maximum automatic runtime for safety and energy reasons.
FIX: There's no setting to change this; it's a built-in limit. If you need to keep food warm longer, you can manually restart the 'Keep Warm' function once it turns off. Just press the 'Keep Warm' button again. The display will show the timer starting over.
IF THAT DIDN'T WORK: If it's turning off after just a few minutes, not hours, there might be an issue with the temperature sensor. Make sure the inner pot is correctly placed and that you're not using a 'Keep Warm' setting meant for something like yogurt (which is a lower temp).
HEADS UP: For food safety, don't leave perishable food in the 'Keep Warm' mode for extended periods beyond a total of 4-6 hours, even if you restart it.