As we learned earlier, in order to display metafields on the storefront, a custom Liquid code snippet must be added to a respective template file of your theme. That's a nice point to start - you may want to make sure that there is such a code in your theme. Checking the official documentation for your theme or contacting theme's developer seem to be the best options to get this information. 

The next thing to check is whether the "missing" metafields match the Liquid code pattern set in your theme's code. The best way to find out what's wrong with specific metafield is to find another metafield (the same metafield, but from another product, for instance) which works fine and compare their main attributes:

1) type 

2) namespace 

3) key 

If none of the options above worked for you, you may want to check whether the product with missing metafields use the same theme template as the rest of your products.