Did you put a K-Cup in before adding water? Guilty as charged. The machine might start, panic, and then give up. Always add water to the reservoir first, then insert the pod.
Also, check your cup size setting. Some Keurig models have a button for 6 oz, 8 oz, and 10 oz. If it’s accidentally set to the smallest size, you’re getting a shot glass of coffee. Press the button again like you’re correcting a typo.
Keurig K-Supreme Plus Not Brewing Full Cup: Troubleshooting Tips and
And yes, your mug might be too tall. If the cup sensor gets confused, it thinks your mug is a dwarf and stops early. Try a shorter cup, just to rule out mug shaming.
