I'm murdering all my onion seedlings
I put a red and a white variety of seeds in a propagator on the windowsill in my conservatory and when the first ones sprouted I let them get to about 5cm above the soil and moved them in to pots. The ones in the conservatory died, the ones put out in the garden died. The next seedlings I got through I left in the propagator, made sure there was water in the bottom so they wouldn't dry out, and they're dying. What am I doing wrong?