Some things I have found as I cruised the internet looking for a solution to this problem:

1) Most commonly, the upper puncture needle becomes clogged. This needle pierces the top "paper" in the center and pushes hot water into the K-cup. The solution is to run an unfolded paper clip up each of the three channels in the needle to clear the blockage.

2) Also referenced is a clogged lower puncture needle. This needle puts a hole in the edge of the plastic "cup" to provide a drain where brewed coffee can exit the K-cup.

3) Calcium build-up in the lines can restrict flow. Periodic descaling with vinegar is the fix for this build-up. Run several reservoirs full of plain water to purge the system of residual vinegar.

4) The exit from the water reservoir may be clogged. Taking the plate off the bottom gives access to the drain and filter. It is easily done just, need a screwdriver and a pair of pliers to take a pinch clamp off tubing.

5) It has been reported that a resounding blow or two to an upside down Keurig has started good flow.

6) Air pressure from an air pump drives the hot water out of the boiler, through the K-cup and into your coffee cup. A failing air pump can deliver a cup less full. There is a very simple way to lubricate the diaphragms in the air pump with vegetable oil. Replacing the air pump is not so simple and requires significant disassembly of your Keurig. https://youtu.be/2A_Rq1jxgwc