I have both briars and cobs; one thing I noticed a long long time ago about the briars (mostly Savinelli's with fairly evenly drilled bowls) is the tendency for there to be air gaps toward the bottom of the bowl that can occur while packing--everything looks and feels fine on top though...why it tends to happen in these and not as much in the cobs, I have no idea. That said, after packing like you usually do, take the pin of your pipe tool and poke it down through the tobacco in the bowl, if it goes in easily after the first 1/4" or so, you've got too much of an air pocket there; to fix it you can do one of two things: either repack it, or use the pin to work the tobacco down into the air pocket.
If, if, if this is what is happening to you (which I don't know if it is or not), when you pack your briars, pack very small amounts at first packing down (lightly) as you go. When I pack mine, I pack from the pouch putting in only about 1/5 of the total amount each time, tamping down with my little finger as I go (and yes, I know I just gave some on this site a heart attack, but you try packing while working a trolling motor in the wind! packing from a pouch with my finger is how I do it.)