When making pour over coffee, why does it sometimes take a very, very long time to drip out of the filter? 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?
The problem could very well be what Stephie was touching on; very fine coffee dust restricting the flow.
I was having the same issue with my setup and was just going to live with it. That is, until I took off the hopper off of my grinder and saw really finely ground coffee stuck between the splines of the burrs. After brushing out the powder and a couple firm taps it was back to normal.
Depending on the different beans along with their level of oil, grinds can build up on the the burrs and internals of the grinder rather quickly. After a bit of time the caked on and powdered grounds give way and fall down into the stuff you just grounded. When you try to brew a cup, the smaller lighter coffee grinds/dust flow down to the surface of the filter restricting the flow by blocking all of it's pores.