Dahlias from seed (No replies)

Can anyone please advise.  Three times I have tried to grow dahlias from seeds in a covered seed tray in a propagator in my greenhouse.  The pattern is after a few days little delicate seedlings appear - great.  The following day they have ‘evaporated’ - gone without trace.  I keep the tray watered.  Any ideas what’s going wrong?  All other seeds I grow in the same way seem to germinate and develop normally, but not dahlias.