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Are you using any page builder plugins or custom fields for content entry?

Reading Time WP works by counting the number of words in the post_content. Custom fields typically store their data in postmeta. There’s such a wide variety of fields that could be stored there’s no way to catch them all. Here’s a note on how you can customly add fields https://wordpress.org/plugins/reading-time-wp/#how%20can%20i%20add%20meta%20fields%2C%20say%20from%20advanced%20custom%20fields%2C%20into%20the%20reading%20time%20wp%20count%3F

Then you’ll want to take a look at using the rtwp_filter_wordcount filter to add your ACF fields to the wordcount. https://wordpress.org/plugins/reading-time-wp/#how%20can%20i%20add%20meta%20fields%2C%20say%20from%20advanced%20custom%20fields%2C%20into%20the%20reading%20time%20wp%20count%3F

Another example from the support forums of how a user integrated with their ACF fields. https://wordpress.org/support/topic/include-word-count-for-acf-fields-cpt-and-multiple-templates/