custom thumbnail sizes in functions.php don't show in size drop down

None of the custom thumb sizes (thumb-small, etc) listed in functions.php show up in the size drop down after uploading an image. The only size that shows up is the full size. I’ve got the image sizes in WordPress>settings>media all set to zero. If I set them to the same sizes as those in functions and then upload, the sizes are there, but for some reason are not being read from functions.php when set to zero in WordPress.
The images are created in the directory, they just aren’t visible in the drop down. I’ve tried deleting the theme and reinstalling and also deleting and reinstalling wp-admin and wp-includes.

I added the following code to the functions.php in the Hueman Child Theme and the “New Size” image showed up in the drop down, but not any of the other sizes from the main functions.php file.

if ( function_exists( 'add_image_size' ) ) {
add_image_size( 'new-size', 350, 250, true ); //(cropped)
}
add_filter('image_size_names_choose', 'my_image_sizes');
function my_image_sizes($sizes) {
$addsizes = array(
"new-size" => __( "New Size")
);
$newsizes = array_merge($sizes, $addsizes);
return $newsizes;
}

Any suggestions?

Also, when adding existing images to a post, only the thumb, large and full are listed, despite the fact that there are multiple versions of the files from the previous theme.

Any help would be greatly appreciated.