Plants often show signs of stress 3-6 days after the actual problem occured. You should be using VERY LITTLE nutrients if any while cloning. Just use distilled water for a week then if you're really ambitous use a fifth of what the nutrients say on the back for the rest of your cloning. Another thing that might help is a sugar refracto-meter, it measure the sugar content or something and can indicate plant stress before the plant shows signs. Oh, and another possibility is light stress, back your light up just in case if it's anything besides a flourescent.