Antirrhinum seedling problem
 

I sowed a packet of antirrhinum seeds into commercial compost, then
placed the tray in a heated propagator in the South facing window of my
garage. I had an excellent germination rate, after a few days removed
the glass cover, then a few days after turned off the heat. About a week
later I notice a number of the seedlings had no leaves, just the stalk
remaining. the other seedlings remained healthy. However the number of
damaged seedlings are steadily increasing, so that now over two thirds
have gone. There is no sign of any dead leaves nor any slug trails,
though after seeing the first damage I did place a few slug pellets in
the tray. Any ideas of what the problem is? I would like to sow another
tray, but not if I cannot resolve the problem fist.
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Residing on low ground in North Staffordshire