There are three reasons why teams usually score few or no goals when the game is already won at half time:
1) It's considered unsporting to 'run up the score' by humiliating an already beaten side
2) Making attacking runs and drawing last-ditch challenges drains stamina and risks injury
3) Teams tend to go defensive when the match is already lost, to keep the score respectable
1) and 2) you can basically do nothing about. You can sub on youngsters or someone who might be out of the team and have a point to prove, but in my experience this rarely leads to goals as they are still subject to 1) and 2), and it doesn't change the outlook of the other nine men. You can do something about 3), e.g. by pressing up as Logorex suggests, maybe playing wider, pushing the full backs up to get crosses in the box and trying to break them down.
And you could always try giving your team the hairdryer treatment at half-time even though they're playing brilliantly and seven up. It will almost certainly **** off the entire room and require some major ****-kissing in your office afterwards, but you never know, some of the players might go for it in the second half.
If you seriously want to win 14-0, you need a much bigger mismatch than you'll find in the German Cup. I recommend either PSG, where you might face an amateur side in the French Cup, but only one or two a season. Or a Brazilian side, where you get some right dross in the state championships, and plenty of games against them. Internacional had a 17-0 win in the Campeonato Gaucho in my save.