What are your custom tactics? I play on 6 depth 4 width and pressure on heavy touch and my defending is extremely strong at the moment. I've kept 7 clean sheets in my last 9 games and one of the games I conceded in it was a penalty. There's no doubt that it helps to have excellent defenders as well. I'm using Van Dijk and Scream Pique and they're an absolute dream to use. I would highly recommend investing coins in some top quality CBs and also ensure that you're switching to manually control them otherwise they likely will just stand there.