Keurig Brewers and many other coffee brewers are sensitive to water used for brewing. With Keurig, I recommend using either purified waster (distilled or reverse osmosis). Many bottled waters, filtered water and tap water can leave minerals in the water tank that will eventually cause your brewer to need descaling or stop working correctly (short cup or no brew).
I ran across a YouTube video that I have not validated that shows one potential fix (below). Also below is a copy of the Keurig generic descaling instructions for home brewers and a link to the Keurig documentation website.
Here are links to the brewer cleaning and use and care instructions for Keurig Brewers as well as a link with info for a brewer not brewing issue.
Hopefully you find this information useful.
Generic Descale Keurig Brewers information.
For Use and Care Guides, Quick Start Guides for all Keurig Brewed machines, please visit the Keurig documentation page: http://www.keurig.com/Customer-Service/Brewer-Registration-And-Support/User-Guides-And-Help