How can I solve the filling problems of my Keurig? My Keurig works perfectly when I don't have a K Cup in it (just plain water).  As soon as I put one in, it runs slow and does not fill any of the cup sizes.

I have descaled and cleaned out all the needles, how can I solve the filling problems?
You need to descale the entire path of the water takes.

Our office Keurig suffers the same symptoms occasionally.  When they occur, we use the Keurig descaling solution and the instructions located on the Keurig Website.

You may be able to use vinegar and water if the instruction for your model state that it's OK.