This is probably caused by the use of tags and shortcodes in the text.
To read a post or page clearly, we strip all HTML tags and shortcodes from the text. Unfortunately, Wordpress includes any text between shortcodes (our own [responsivevoice] [/responsivevoice], for instance) as part of the shortcode itself, and it thus removes it. Note that this does not impact your text in any way, it's just internal processing.
Let's look at an example:
Roses are red,
[responsivevoice]Violets are blue [/responsivevoice]
Sugar is sweet,
And so are you."
While the text will appear like you formatted it, [responsivevoice_button] will skip the "Violets are blue" line, as it is included between shortcode tags. [responsivevoice] will work normally.