I may be wrong but I guess in your case the theme for which your plugin is not displayed has the particular action removed. (you may check that in the respective theme's functions file.) Well, it is better idea to hook your plugin (functionality related into some action hook which is related to functionality or which renders some funcitonality (rather than being an UI related action).
Like, the action you have used to hook your plugin
woocommerce_after_main_content along with another action, only outputs an html element wrapper. (which some themes might remove and use their custom html wrapper.
In your case since your plugin is related to / displayed on single product page, I would recommend to use relevant action to hook your plugin. e.g.
woocommerce_after_single_product, which are generally not removed by woocommerce compatible themes.
Hope this helps.