When using Shopify you might notice that products expire. This is because products need to be submitted every 30 days.
There seems to be an issue with the Content API used by the Google Shopping app within Shopify. And both parties will tell you to contact the other party to resolve, forcing you in an endless loop.
One option is to go to the specific product that has expired in Google Merchant Center and edit something, like the title, description, etc. this will then force Shopify to post this new data to Google Merchant Center. Your products are now active for another 30 days.
Using a data feed that updates every day for all products. Such as using a text delimited or XML data feed.
For my clients, I use datafeedwatch, which allows you to set a daily fetch, which means your products will never expire. You can also set Google Merchant Center to collect the product info every week.