A mysterious problem regarding functions.php and image size

Thank you for the response. Here is the theme link:

https://wordpress.org/themes/smartline-lite/

There is no problem in the offline version on my laptop; only the online version has this problem. Both have the same theme and WP versions.

The image sizes are set in functions.php like this:

// Add custom Image Sizes
add_action( 'after_setup_theme', 'smartline_add_image_sizes' );

if ( ! function_exists( 'smartline_add_image_sizes' ) ):
function smartline_add_image_sizes() {

	// Add Custom Header Image Size
	add_image_size( 'custom_header_image', 1340, 250, true);

	// Add Featured Image Size
	add_image_size( 'featured_image', 300, 200, true);

	// Add Slider Image Size
	add_image_size( 'slider_image', 880, 350, true);

	// Add Frontpage Thumbnail Sizes
	add_image_size( 'category_posts_wide_thumb', 600, 240, true);
	add_image_size( 'category_posts_small_thumb', 90, 90, true);
	add_image_size( 'category_posts_single', 880, 260, true);

	// Add Widget Post Thumbnail Size
	add_image_size( 'widget_post_thumb', 75, 75, true);

}
endif;

Also, I noticed that when I change the size of the featured image in the above code to less than 90 x 90, the changes are reflected. Anything bigger than 90 x 90 doesn’t show up.

sorry i dont know much about this or php. So what should i do to fix this problem?

where can i do this?

Thanks….
Any idea why it works offline on my laptop running MAMP, but not online?