The variegation is random on the leaves so you cannot prevent or enhance it much. I would recommend providing slightly less sunlight than usual (say 2 hrs or locate it in bright shade) in a special
planting location but, other than that, you cannot "prevent" solid greens or force more variegation by doing or not doing something. Some variegated shrubs I have seen for sale at Lowes showed a sliver of variegation, others showed more variegation than others and yet others showed a few green leaves. They do best in more shade so see if that helps.

The process is called reversion. This often happens when variegated plants mature, and eventually the whole plant can and may become solid green. Scientific research suggests that the process begins when there are environmental conditions that bring about a chlorophyll deficiency in the plant, which causes it to attempt to compensate for this. I do *not* personally believe that reversion is a reversible process.

Conversely, others have reported "albino" leaves on some hydrangeas. Although in the long term, albinoism in most plants is NOT sustainable as the lack of chlorophyll in the leaves provides zero nutritional advantage to any plant.

Other examples are how Fire & Ice hostas eventually revert to the standard Granddad hosta once the bulbs have aged.