Some of my posts are missing the "Read More" button

Ah I see. It loses in precedence.

Try this:

#wpsp-3004 .wp-show-posts-entry-summary p.read-more-button-container a:hover, 
#wpsp-3004 .wp-show-posts-entry-content p.read-more-button-container a:hover{
    color: white !important;
}