I think the trouble is all the producers are trying mixes that include lots of wood chip to replace the peat in the old mixes, and they have very little nutritional content. That's ok for seed germinations, most of my seeds germinated, but after that growth stalled until the baby plants give up altogether.
Even in very poor sieved soil, my most recent batch of seedlings have done better
It worked ok because I could tell the difference between brassica seedlings and the weeds that were in the soil that also germinated, so a far from ideal situation for everything..
It did however prove my point, that seed composts are pretty useless this year