The Senseo has started to produce only half cups of coffee - using the same pads, and nothing appears wrong with the machine.

SOURCE: Senseo machine has started producing very weak coffee

I don't know if you've tried this, but it works like gangbusters for me. I use non-senseo pods with the 2-pod holder, and they're too big for the 1-pod holder but not quite big enough for the 2-pod one.

I would frequently end up with a lousy, weak cup of coffee, and after a little investigation I deduced that water was flowing into the pod holder and AROUND the pod, without going THROUGH the pod directly. The name-brand senseo pods fit perfectly, and always produce a strong cup, even if I don't like their coffee, and it's because there's such a tight seal between where water flows in and where it flows out.

Simple solution: Put your pod in with the flat part facing up, and the bulgy part facing down. Press it down hard into the 2-cup holder. Run a little HOT tap water, and lightly wet the surface of the pod--not so much water that the pod starts floating in the holder, but enough to get it soggy. Then make your cup of coffee. The water will flow through the pod first, and kind of like siphoning gas through a hose, once it's started going through the pod it won't stop. Instant awesome cup.

SOURCE: philips senseo coffee is watery

It may be the pod holder,there is a seal under the black disc at the bottom of the holder,the water will bypass the coffee under pressure,travel to the outside of the pod holder and exit the centre where the filter mesh is,if there is no foam also this is a sure sign,fill the empty pod holder with water and see how quick the water drips,I had this same issue and bought a new pod holder and compared,should be no more than 1 drop a second,you can knock the plastic out and re seat the O ring or buy a new holder,hope this is of use to you