Most variegated plants require full sun or dappled shade to maintain their variegation and vigour. The paler parts of the leaf lack the chlorophyll needed to create energy for growth. So plant in a sunny situation to make full use of the chlorophyll they have. This principle does not hold true for all variegated plants but needs to be considered.
Sometimes variegated plants will revert back to their original plain green colour; this is known as reversion. Prune out the reverted branch or shoot before it takes over and affects the overall appearance of the plant. It could be a sign that the plant needs more light to maintain the desired variegation.