Always check with your vet for an underlying health issue that may be affecting your cat's appetite. If nothing is physically wrong with your cat, it's most likely a preference issue.
Cats can be picky! They may get bored of a food they used to like. Post Consumer Brands makes cat foods with a variety of flavors and textures to help you mix things up.
You can also try moving your cat's bowl or dish to a more quiet and out-of-the-way location where your cat can more easily see you. Putting a toy in your cat's food bowl may also help keep his or her interest and make eating an entertaining activity.