This is just what im thinking----How are you getting into your stand location,example--walking through their bedding area? you said it was real thick around the opening,maybe you are over hunting the same stand location and the bucks have you patterned.
If a buck feels secure in his domain and there is an opening in the middle of the bush surrounded by thick stuff my guess would be that he is hittiting it at night.Could be to much scent or movement on your part also.