I had this problem and called the Keurig support number. They helped me fix it. Your tool bag for this project is a paper clip.

First, take out the plastic holder that holds the K-cups. You should be able to turn it a little and it just lifts out.

Once out, take that piece apart. It easily snaps together and apart.

Once it's in two pieces, you see the bottom of the small metal tube that pokes the hole into the bottom of the K-cup. Take the paper clip and insert it into the tube and see if you can dislodge any tiny bits of coffe, tea, or whatever you've been drinking. You can run water over the tube while you do this.

Second, lift up the handle in front all the way up. If you look under the top, you see the second set of tubes that inject the water into the K-cup. There should be three of the tubes. Use the paper clip and clean them out as well.

Put it all back together and get a glass measuring cup. Run three batches of water through the machine without using a K-cup. Each time, note the amount of debris in the water, and note how much water is being put into the cup. You want the water to be almost clear and the amount to be consistent.

If this doesn't work, keep calling Keurig support. You have a 1 year warranty and if it won't work, they will eventually agree and send you a new machine. (Assuming you have your dated receipt.) My brother got them to send him a new machine because his stopped brewing too. They didn't want it back so he decided to take it apart to see what the problem was. He said he had a split water line, so in that case, no amount of pipe cleaning was going to help.

Good luck. I use my Keurig every day and really like it.