over watering or lockout by salts not over ferilizinf by nutes or you would see burn or deficiency let them babies get good n dry before u water em again
Looks like you might have planted straight into a hot soil tho? if so that ould also be nutrient burn setting in as those plants are to young for many nutes...
Could be disease/pest or slight cal mag issue its so young its really hard to tell i think i would isolate it for a week from any other plants and update ne with a pic in a week and if it is still having problems ill help identify them for you a great read if you havnt already is...
I agree with the first post you do not need a dome for seedlings only clones... From the sound of it since you were "digging up" your seeds to check on em you might be a tad bit impatient and patience is the key with growing marijuana esecially in the seedling/vegatative stage of growth. just...