I rewrite it for the umpteenth time!
The Featured Image in a wordpress theme must be defoult and global ONLY in articles and pages!
The Global Image Featured in all of wordpress pages is a mistake because you can not use any plugins such as: bbPress, BuddyPress, the event calendar and so on.
The only solution to solve this problem in your theme is to disable the featured image in the theme settings and then enable it for each article, but if you have a blog with many items (I have more than 400) is a long job!
I repeat, the overall image featured in all of the theme pages is a mistake!
I do not understand what you do not understand this thing!