Hey Smokinjohn,
Maybe try mixing the real fine dust with some of the shards and loosly filling the sieve...I just scoop up some out of the bag with the sieve, flatten off and even the dust out then dump out a couple of inches so I have the start and finish points.
As your draft is OK, I'm guessing that maybe it was a combination of the dust being really compacted and maybe not being lit enough (good English there!) or even the dust was slightly damp...not enough to feel damp but enough to stop combustion? But give it another whirl and hopefully it'll all work out for you
