problems with alt text not appearing in source code

The display/use of various <img> attributes is up to the theme code. If the theme code does not use the info from the media item, then it will not display.

Fixing this will require digging into the theme code to figure out where the <img> code is generated, and then modifying that code.

Note that directly modifying theme code is not recommended, as a theme update will overwrite your changes. Best to use a Child Theme to modify theme code.