I just had this exact problem ??
Here is what helped me:
Change the actual image size:
add_image_size( ‘thumb-small’, 160, 160, array( ‘center’, ‘bottom’ ) );
to
add_image_size( ‘thumb-small’, 161, 161, array( ‘center’, ‘bottom’ ) );
or delete all images named IMAGENAME_thumb-160×160.png (I guess you already have those)
my guess is that regenerate thumbnails checks whether there is an existing image and does not overwrite.