Seedlings failing again - Help needed

I am a new veg gardener and and struggling with seed germination and the quality of seedlings. I consistently see either very few seeds germinating or the ones that do have very weak stems that wither at the base and snap off (picture attached of purple sprouting broccoli). I am making sure they are not planted too deep, that they the soil is moist but trying not to over water (I have them in a propagator), and trying to give them plenty of sun. Any help/advice on where I am going wrong would be greatly appreciated.
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Tell us more about them. Did you dow them individually into modules or have they been pricked out? If so I think they’ve been pricked out too soon. They’re too small. They need more leaves and roots to cope with that.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Not heat and sun. Light.
I live in west central Scotland - not where that photo is...