My Keurig (100 series) started doing this rather suddenly. Once day it was fine and then over a few days it suddenly started brewing 3 oz cups. The worst decrease in performance came when it had just gone unused for 3-4 days (I was away). Before that I was getting 4-5 oz cups. The coffee also isn't very hot, more like lukewarm.
I use filtered water and brand kcups.
Maybe these clues will point out what my problem is. It seems like any kind of build up or blockage would happen gradually over time, not all of a sudden in a few days.
One tip for those with this problem: keep running the Keurig through its cycle with the same kcup. You'll eventually get a full cup of coffee but it takes a while. Then you have to put it in the microwave to heat it up. Temporary rigged fix.
Thanks for any help.
Marsha