The only thing i can think of is their positioning. They might have too attacking mentality so they don't position themselves right compared to the striker.
Another thing is they might be outpaced by a wing attack and aren't in time to prevent the striker to head the ball into the net.
Also i might have to do with the positioning of your MC's. If there's to much room between your defense and midfield it might be the room for a opponent to head the ball your CD wasn't able to reach.
Besides that, i can't think of anything. Besides the logical explantion the opponent have even better heading/jumping/length stats, but that's something for you to sort out