We have set up our new store and despite emailing Google and their technical teams, they simply will not reveal / pinpoint what the issue is.
Any ideas where we are going wrong?
Sorry to hear, it has to be incredibly frustrating.
Sometimes it's something as simple as promoting a product that is now out of stock. Google put out a full list of why they mat have flagged your store, I'd read through and fix anything that you think may have caused it and then request a re-review. Here is the list.
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Best of luck!