you dont have them in high humidity do you, i find you only need high humidity for clones, seeds dont need it and it would probably keep the growing medium to moist and rot the tap root, as long as the medium is slightly moist then the roots will grow with no problems, if its to wet then the tap root dont get any oxygen and it just slowly dies, or if it grows then the plant will be weak till it has enough roots, i find that spraying the growing medium once a day with a fine mist is just enough to keep it moist without drowning the seeds,
let me know how you get on
let me know how you get on