Some things I've noticed about lump:
It will pick up moisture easier, doesn't have to get rained on, may just be in close proximity to a heavily watered lawn.......
Will have a wider range of temprature, but needs MORE air at the minimum to keep burning (see below )..............
If some is not as well lit as the rest, needs better contact than briqs. to keep the burn going........
Some things I've noticed about briquettes:
Will burn as long as lump, but not at as hot a rate......
Will continue to burn with less contact between pieces, most probably because of the binders that hold it together into the original shape....those binders are a lot of times designed to keep things burning.....
My best wild guess on this particular problem would be that it's not getting quite enough air, possibly along with not quite being well lit enough at the start......let it go all the way to ashed over & as G says, add a bit more in & "shake" it a bit before you lite to get good contact in the chimney.......
Not sure what to do if it's not getting enough air, maybe just make sure it's loaded into the cooker as close to the intake vent as you can get...?
Hope this helps..........
If you have room on the porch, patio, area.....maybe get an 18" or a 22-1/2" Weber also....Garage sales are great & CraigsList in your area.....a lot of mine have been cheap or free & you can do an amazing amount of stuff with even an 18"....these larger ones are easier to get the airflow going & might only cost you $10 to get one !!!