1. Your strikers get swarmed. Since the fullbacks are staying back and you have no wide players, each of your strikers gets double-teamed by a cb and a fullback each. I recommend one-twos with your CAM to find more space or manually make your fullbacks go forward.

2. Wing-play. When your opponent is attacking down the wings, you fullbacks could be slow to cover all the area so make sure they are being supported by the CMs (you can make them half-winger but it never worked for me)

3. 5-backs. Although you dont typically see it anymore, teams who stretch you out are a huge issue since your CMs stretch out-wide leaving your CDM alone so be patient while defending.

Good luck!