The div to which the scrollbar wasnt being applied was hidden by default, and as per the documentation, it would not be visible in such cases. The solution – modify the jquery as per the documentation.
> n order for the custom scrollbar to work correctly, your content block must be visible (i.e. should not have display:none) and its content should be fully loaded. Otherwise you must call plugin’s update method after the above conditions are met.
> Hidden content blocks (either by CSS display property or jQuery $.hide()) have zero dimensions, so calling mCustomScrollbar function on them will not work. Instead, you should call the function after your elements become fully visible, so the script calculates content length correctly and apply/update the scrollbar