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.
I typically clean with vinegar and water but may I make a suggestion... ditch the Keurig!!
I have heard that if you overfill custom k-cups grounds can get pulled into the water supply lines that may be a problem, but I will be honest, I feel like Keurig brewing methods significantly degrade the flavor of your coffee. I will admit that I own one, but I would choose a drip brewer into a thermal carafe any day over a Keurig, and they tend to have much fewer problems like overflowing, not brewing a full cup of coffee etc. etc.
As to your actual problem, it is probably grounds obstruction, scaling, failing pump, or an over-filled k-cup. Descaling may help, otherwise it's either brew lighter coffee or replace the machine. Unfortunately not a lot of good options.