Adding items to cart from related entry not working
Just quickly as I'm on my phone...The two main ways to debug this sort of thing in Commerce are:


Check what you are actually POSTing matches what you think you are posting (to check this I just use the browser dev tools)
Check the flash data (notice and error) for messages when the page reloads (or check the JSON response if ajaxing). 


Usually you'll get a sensible message back, e.g. out of stock or similar.  

I use this code to dump the flash data (I think only error and notice are used these days, the commerce flash dates back from before commerce was released but it can't hurt!):

{% if craft.config.devMode and (craft.session.hasFlash('commerce') or craft.session.hasFlash('notice') or craft.session.hasFlash('error')) %}

        {% set isErrors = true %}
        {% set commerceFlash = craft.session.getFlash('commerce')|join('|') %}
        {% set noticeFlash = craft.session.getFlash('notice')|join('|') %}
        {% set errorFlash = craft.session.getFlash('error')|join('|') %}

        Commerce: [{{  commerceFlash }}]
        Notice: [{{  noticeFlash }}]
        Error: [{{  errorFlash }}]

    {% endif %}