Keurig water pump stops in just a few seconds

My Keurig is a K-Mini, not exactly like this one but very similar. It will make one cup usually and if you try to make a second cup it will only pump just a little bit of water and then the pump stops. If you open the K-cup lid and close it again, then push the button it will pumpe for a second or two and stop. Repeat this process enough times it will finally get enough water in it to start heating up.

My question is why does the pump stop after only a few seconds?