Basically, the Starclan is just the territory clans hold within the Ethereal Plains, and without collective support from the living - belief and energy - it's easy for souls to fade away and be reborn/recycled into something else. Either reincarnating in the material world or being turned into something different within the Plains (Cottonkit chose to reincarnate! right now Berrypaw manages that as the Starclan guide). Certain cats are born with a natural intuition and connection to the Plains, and depending on the strength of the spirit can communicate or have ghost sight, otherwise best form is through the dream bubbles (if anyone says homestuck I'm blocking you /j)