There is a little pin at the base that punctures the pod to let coffee out the bottom. If that is broken or missing the pressure from above ruptures the top of the pod and blows grounds out to the cup. If indeed that is the problem, the pod will have a big jagged hole instead of a clean puncture. If you are using plastic refillable pods occasionally, be sure that they are rotated to allow it to drop fully. If not, that is what breaks the bottom puncture pin.