Could be damping off or root rott. Is it fresh soil or re-used? The soil could be too wet. A very mild bleach solution (like 10 to 1) can be used to prevent damping off, if that's what it is. Even if it's not, it couldn't hurt. Do you know what the soil temp is? You could also try a seedling heat mat to warm the soil above air temp. They're not that expensive and it will help propagation. Or you may have better luck starting them in rockwool or plugs