Why has the product been on Google Merchant Center but ads are not displayed?

In this case, Shopping ads are not displayed due to some of the following common reasons

The product is not ready for distribution
It takes about 72 hours for the first product synchronization and 24 hours for the sync times and then for Google to review your product. Products are eligible to run Shopping ads when:
+ On Google Ads:
Product status: Ready to distribute
+ On Google Merchant Center:
Advertising Status Shopping: Active

The product is Active on Google Merchant Center but not ready for distribution on Google Ads
Your product is out of stock. You provide the availability of the product by using the attribute "Availability" in your product data. If this property of the product is set to 'out of stock', the product will not be ready for delivery. When the product is stocked, make sure to update the product properties to show that the product is currently available. 

The product is ready for distribution but not displayed due to reasons:

Bid price of the product is too low. Consider increasing bids

Campaign budget is limited. Consider increasing the budget

There are very few user searches that are matched. If there are few user searches that can be matched, consider adding more details to the title and product description.

Low demand. Demand for products is low so ads are not displayed