Rule #1: JACKSON'S ARE SHY
Jackson's chameleons are the most unsocial and difficult chameleons out there (other than maybe brookesia). I had one for quit a while, but he was a pain to take care of. Other factors for your cham's shyness:
what is the cage temp? humidity? basking spot temp(usually overlooked but important)? visual contact with any other chameleons? too much handling? any other possiblities of stress? when the animal becomes dark and more drawn back than usual, it is likelybecause of stress and/or cold temperatures. Is it active? does it say near/on the ground? oh, and one suggestion. If you have a fake plant that is fairly strong, try putting the cham in the plant and putting it in the shower for about 10-20 minutes with a low water pressure using fairly warm, but not hot, water. stress is a huge problem in chameleons, and more so in Jacksons's than most others. and one last thing: how old is the cham? thanks.