1. Train defending set pieces alternate with defensive positioning for about a month on AVERAGE setting.
2. Watch matches and see what kind of crossess you concede to: from fbs or wingers, drilled or floated. Then look at your tactics and try to cut the crosses out eg by telling your own wingers to man mark opposing wingers, your fbs/wbs not to attack so much etc.
3. If crosses are floated even in set pieces then check the hight of your cbs.