Compost differences

I've been mostly using B&Q multi-purpose compost (had a tiny bit of leftover stuff from another brand) for all my sowings and potting on. It's seemed okay for most things but I'm having terrible luck with my brassicas. They are either not really germinating at all (though that may have just been that the kohlrabi seed I have is no good), or germinating beautifully and then suddenly dying a few weeks later. Very frustrating. I have lost all my cauliflowers this way (they had been looking great) and now most of my cabbages are going the same way. They are indoors, so it's not that they have been frosted.