could i ask what type of light you are using? clones are pretty resiliant but very delicate at an early age especially if they have small root balls. ive grown thousands of clones over the years and 600w seems a bit overkill for a 3 - 4 week clone IMHO. from my experience its best to use a light with lots of blue and red spectrum for a few weeks to help get the veg going and the root system then once they have got used to the light use something a little stronger. loking at the size of those clones they look far too young for a 600w. im not saying this is your actual problem but it sure maybe a contribution to it. ive also found that rushing clones into strong light before they are ready can effect the final results. hope everything works out for you and best of luck good buddy.