small seeds

I am having trouble growing plants from small seeds. 3 packs of different variety of Leeks, 1 of Celeriac  and 2 of  Cabbage. The packs are new and the seeds have been sown in 3 instalments by different depths using wilko and JI no1 seed soil.

The large seed broad and dwarf beans, mini pop corn, and courgettes have all grown well.

The Tomatoes and early Brussels are doing well and they have had the same water as the 3 types in the first paragraph.

Any suggestions or hints and tips will be warmly received.

I understood (perhaps wrongly) that a reasonable rule of thumb is that seed should be covered by about as much as the size of the seed itself so very small seeds either need no cover or very very light cover.  there's also the light issue of course.

Adri

thanks for the input guys

I find this method works well for brassica seeds:
Take some multi-purpose compost, add about a fifth by volume of perlite or vermiculite and a light dusting (like icing sugar on a cake) of lime. Mix well.
Fill a half seed tray or a small pot and firm it slightly
Water well and then drain (I float the half seed tray in a full tray of water)
Thinly scatter the seeds on the surface and then lightly pat them with a finger so the seed is in good contact with the compost - not pressing it too hard though.
Thinly cover with vermiculite - just enough that the surface of the compost is hardly visible.
Place in a propagator at 20ºC - optimum germination rate is achieved between 10º and 25º
Once the seedlings are about 3cm high, prick out into 3" pots. I like a mix of 50/50 by volume of multi-purpose and John Innes No2 with a little lime added. I plant deeper than they were in the seed tray, about half way up the seedlings stem.
Harden off gradually and plant out when a reasonable size. If club-root is a problem, it can pay to move into a larger pot to allow better root development .
There's information that will help here:

Optimum Germination Temperatures

Growing Brassicas