The only way I know how, is by checking and re-checking the policies, see what potentially are the reasons, and fix them. It’s a big process, so I recommend checking the below policies:
https://support.google.com/merchants/answer/6150127?hl=en
https://support.google.com/merchants/answer/6363310/follow-the-merchant-center-guidelines
https://support.google.com/merchants/answer/7052112
https://support.google.com/merchants/answer/4752265
https://support.google.com/merchants/answer/13693865