get some sort of antiwilt and take the domes off, you could also be retaining too much moisture in your rockwool, although I do agree that yellowing leaves on clones is normal as stated above...some of us here do not use heat and domes, but keep our clones in hydro format and use an antiwilt product...personally, I use miracle grow leaf shine, it has a waxy content and keeps the leaves pert and healthy during the rooting process...but you can buy an antiwilt at any hydro site, that is supposed to work really well...Did you use a cloning gel/powder...that is essential to successful cloning...I use schultz rooting powder available @ wally's...it works great, :thumbsup: I have 98% success with these 2 above mentioned products.
hope this helps.