for some reasons quick-adsense is using global variable $ShownAds to count how many ads were already inserted in the process_content function.
Any call from any plugin that for it's internal reasons need to process post text by calling apply_filters('the_content', will trigger that process_content function.
Since $ShownAds is global such calls will increase the counter and when it's time to actually output the post this counter could be 10 or 20. If the limit of 5 ads is set - NO ads will be shown, because the plugin will think that limit is already reached.
Please consider stop using global variables, this is a very bad development practice.