We're having warnings on one site for the product schema. The code has
"brand": {
"@@type": "Thing",
"name": "@Model.Brand"

Google is saying it should be "@@type":"Brand"
https://developers.googl.../structured-data/product

I found another issue that caused all but three of the products on this site to just get kicked off Google. The stock/availability status in the schema is not the same as is reflected in the Google feed for out of stock items.
InventoryManagerData inv = null;

inv is never populated so when you hit if(inv!=null), you don't hit the rest of the code

Also, please check bool allProductOptionsSelected = (optionChoices != null && optionChoices.Length == product.ProductOptions.Count);

allProductOptionsSelected is coming up false for products with no options.