I regularly make coffee using a ceramic #2 Melitta-style cone and natural brown filters. Most of the time it works great and consistently makes delicious coffee. But every now and then, the coffee takes a *really* long time to drip through the filter and out of the cone. More specifically, it normally takes a few minutes for it to fully drip out after pouring on the water, but in these cases it takes in excess of ten minutes. If I lift up the cone, I can see it's only letting out around one drip per second. I often have to apply gentle pressure to the filter to get it to actually drip out the coffee.

I haven't found any rhyme or reason as to why this is happening -- changing the size of the ground doesn't help nor does changing to a different coffee. The only thing I haven't changed is trying out a different brand of coffee filters. Could this just be inconsistencies in the production of the filters themselves?