I have the same rig (Darby) and I'm burning Blaschak nut.
Ash chunks will 'bridge up' some with this coal. I've burned other stuff that ashes to dust and don't have this behavior. It normally happens, as Greg alludes, when you're burning it pretty hot. I use the hardware store magnetic thermometer about a foot out the back of the stove on the flue. In warm times, it will purr along at 200* or less closed down (OAT can be in the 50s). I like 300* as a norm. 350* is pretty hot. I'd definitely get 'bridging' when that hot with that coal.
Greg suggested 'flossing' the grate and that is the simple solution. A piece of wire with a 3" bend at the end works for me. I use the wire from a politician's lawn sign cut to suit (at least I have some use for him). There is enough room to work it over the pan and into the grate bars. Simply floss every couple bars until some orange comes through. Then shake the grate.
Other notes on the Darby. It's a very simple process to use this stove and they work very well - we simply adore it. It is not fussy about disturbing the bed at all and the coals don't mind a 'punch down' if you push or rake from the top, instead of flossing. Just open the ash door and it will come right back with it's jet engine sound and red hot coals.
We load and leave the door open until we see good flame and it sounds 'right' (which is generally 400-500* on the thermo), then close her up. Beware that it will race past that temp quickly and you need to be on your toes. You will tune into it.
Keep her full, from the brick line at the back to the top of the front fence. We shake and load every 12 hours (7 and 7), with the pan being dumped every other loading. A full load will go about 16 hours before there are too few coals to bring it back.
Shake vigorously. The goal isn't to count 6 cranks; it's to get some air channels in there and see some orange. My wife started out doing it by mechanically counting and it caused trouble. Drop some burning bits!
We burn wood above 45* OAT, simply because we have a lot of it around this year. But, it'll burn coal well into the 60s* at a very low rate (I've never found out how high). This might be a function of the chimney. We have a manometer on it and see .06", normally. Being able to 'tune' coal to the need/temperature is so much better than burning wood!