Post thumbnail not showing in WP admin area for custom post type

I think it’s possible the 2 separate enabling statements may be interfering with each other. The purpose of having an array is to combine them into one enabling statement. Try the following in your functions file.

Instead of:

add_theme_support( 'post-thumbnails' ); 
add_theme_support( 'post-thumbnails', array( 'customposttypename' ) ); 

This:

add_theme_support( 'post-thumbnails', array( 'post', 'customposttypename' ) );