Bob Hobden writes
Perhaps you are keeping them too wet, or too cold and wet (humid).
Normally once I've sowed my seeds and put them in the heated propagator
in my heated greenhouse (min 50°F) they don't need watering again until
they are up and growing and then only to keep the compost just moist. I
raise the trays up off the bottom of the propagator so they don't stay
wet from condensation. Alternatively, what sort of compost are you
using? I had a lot of trouble once with a peat free compost (New
Horizons) and cured it by going back to a peat based compost (Levingtons).

--
I don't have any trouble with damping off using New Horizon
despite not being in the least bit meticulous, so doubt it's that. Peat
free composts are said to have improved immensely over the last few
years.
--
Kay