How do I reduce the vertical spacing of Accordion

This may be an issue with the wpautop.

Try removing all space from around your shortcodes. Example:

[accordion]

[accordion-item title="Title of accordion item"]

Dropdown content goes here.

[/accordion-item]

[accordion-item title="Second accordion item"]

Dropdown content goes here.

[/accordion-item]

[/accordion]

Would become:

[accordion][accordion-item title="Title of accordion item"]Dropdown content goes here.[/accordion-item][accordion-item title="Second accordion item"]Dropdown content goes here.[/accordion-item][/accordion]

Ah Ha! This explains some weirdness I’ve seen with other short codes.

I simply ran the closing and opening shortcodes together like this:

[/accordion-item][accordion-item title="Ladies' section" tag="h2"]

and problem solved. Many thanks.

Glad that solved the problem.

You can also remove the weird formatting from all shortcodes by adding these lines to your functions.php file:

remove_filter( 'the_content', 'wpautop' );
add_filter( 'the_content', 'wpautop' , 99);
add_filter( 'the_content', 'shortcode_unautop',100 );

Keep in mind this might have other effects on your content layout so you’ll want to test it before moving it into live site environment.