Like people, cats get constipated if they eat too much of the wrong thing. (For example, it's pretty common for a person to get constipated on the Atkins diet.) A perfectly balanced diet for a cat would be a mouse. They get the protein, the calcium, the contents of the mouse's stomach, etc. It's a mix well suited to what they need. So, what is your kitty eating?
You might try cat lax every day, a small dose.