Google Shopping is a data problem with an advertising layer on top. The feed decides which searches you are eligible for, which products are shown, and whether they are shown at all. Bidding only allocates money across the eligibility the feed has already created, which is why accounts with excellent management and a poor feed stay stuck.
The commonest losses are unglamorous. Titles written as marketing copy rather than as the words buyers type. Missing GTINs or incorrect brand values that suppress whole categories. Variants competing against each other for the same query. Product types and Google categories left at whatever the platform exported by default. And a disapproval list that grew quietly until a chunk of the catalogue stopped serving.
Merchant Center suspensions are their own discipline. A misrepresentation suspension is rarely about the thing the notice names; it is usually about missing or inconsistent policy information across the site, returns, delivery, contact details and pricing consistency between the feed and the landing page. Appealing without fixing the underlying inconsistency usually burns an appeal for nothing.
Free listings matter more than most retailers expect, because they put products into Shopping surfaces without spend, using the same feed. If the feed is right, that traffic arrives as a side effect of work you had to do anyway.