Most commercial mixes are fine, but the addition of some garden soil and compost will often insure adequate beneficial micro-organisms and fungi. No. 1 limestone chick-grit makes a good top dressing for many alpines or slow germinating seed to discourage algae growth. Crushed charcoal also helps. These have hard impermeable shells and need nicking or scratching the coat to allow water to enter and the seed to germinate. The best results are from the least amount of nicking that will allow water to enter and the seed to swell.
Many failures are due to over-nicking and damaging the seed.
Different seeds need varying amounts of nicking. Most do best with a lightly rubbing on sandpaper or a file until just the very outer coat is scratched. Often just scratching with a knife-point or scriber works. Others need serious nicking, often with a hacksaw until the white interior shows. Nicking seed one by one can be tedious but is most effective.