There are times, especially when you change the subject or modified the default size of the thumbnail image, you need to rebuild the thumbnails in WordPress . Can you imagine the task that arises if you have hundreds of entries? What you get lazy?
Because obviously, you saw how to create thumbnails by default when they are not, but we are talking about regenerating thumbnails when, for example, change the subject or modified WordPress default size for thumbnails , where everything descabala and unsightly, not creating thumbnails by default in normal situations.
Nothing but worries install the plugin Regenerate thumbnails , activate it and you get half task done.
From this point you have three possibilities:
Regenerate thumbnails one by one in the media manager.