Some users are complaining about WordPress pagination where the full content is hidden to the RSS feed. This is a WordPress core bug that was reported for about 4 years ago but still hasn’t fixed.
The problem happens when you split WordPress posts into multiple pages and you have set the RSS feeds to display full text. The RSS feed will only display the content of the first page of the post.
There is a workaround for this bug, all you need to do is to add this code to your theme’s functions.php or in a specific site plugin.
This workaround was made by Simon Wheatley and it has worked wonders.
Read the rest of the code here: http://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-post-pagination-hiding-content-in-rss-feed/