I am also having this exact problem and this what worked for me after reading all comments and trying some of them:
1. Go to Products
2. Do the Bulk Editor (click the top most box to select all your products then click Edit Products).
3. If your 'Available on Facebook' are all checked, uncheck all of them then Save.
4. Then check all your 'Available on Facebook' then Save.
5. That should refresh all your product integrations between shopify and facebook.
6. Wait for 15-30mins before Facebook approves and integrates them with Shopify.
7. All your products are now appearing in your Facebook page/store.
Hope this helps!
I had the same problem. A lot of products showed as rejected by Facebook because the product "Must be available for purchase". But those products (almost 60) were all in Active status and available for puchase on teh online store.
I used the bulk editor to uncheck "Available for online purchase" for these products, save it, then immediately recheck the box and save again. This seems to have fixed the problem.
This is a workaround, and thanks for sharing it. However @Shopify: this bug should really be fixed. Is anyone working on it?