I've added more to the culture cup and it hasn't been rejected. More is better and mucus should at least cover the bottom of collection cup. The instructions I got were to get first morning secretions as deep as possible. It takes some work to get the deep stuff out so I know what you mean. One would think that the lighter stuff would show all what's growing in lungs but maybe MAC etc is only in certain places in lungs (nodules, cavities). Sometimes bronchoscopy is recommended to collect those deep samples if the standard sputum sample doesn't pick up anything. That, coupled with ongoing symptoms. Had you had issues with having cultures rejected or otherwise?