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.
Not heat and sun. Light.