I've been struggling with the same issue for a while now and need some fresh eyes on the problem.
Using either a standard Weber 18" kettle or my 18" Jumbo Joe with decent quality Lump charcoal and a large Weber chimney I seem to always only end up with half of the grill burning hot.
Typically what I do is I fill the chimney and I wait until 95% of it is burning good and orange then I dump it and mix the coals real good. There's a few on the very top that are barely lit but my assumption is those will start burning when mixed with the others. I dump the chimney and spread the coals out evenly across the entire bottom grate making sure the unlit ones are mixed well with the lit ones.
The problem is it seems like once I put the cover on and let things stabilize half of my coals go out. Once I start cooking even removing the lid only gets the hot ones hotter and the ones that let me down are just, not lit period.
Is this because I'm not leaving the coals in the chimney long enough, or is it because you can't use the entire surface of the bottom grate? it seems more like a draft problem to me but I'm hoping other far more seasoned guys can help me out.
Still haven't bought a 22" grill but maybe soon, problem is that 26" is looking nice too.