These are things I would consider, if the sun actually shinning on the coleus it may be too hot, indirect light for long periods is very successful. Are you frequently moving the plant to different locations, this can be a problem. Does a draft from a ceiling fan, heater vent or such blow in the direction of the plant? You fed it, unfortunately many plants are "well fed" when we purchase them, if you recently bought the plant and fed it immediately or planted it in potting soil containing plant food the plant may be experiencing a fertilizer burn, oops. They are pretty tough, I would take a small healthy limb from the coleus and try to propagate it in potting soil, no direct sunlight,,,,,, Good luck,