Medical issues in play. When did they last have sex, how long did they wait after. You have to abstain 6 weeks before and after, and until your testing comes back that you are shooting blanks.
Had a friend who went in to have her tubes tied. Pregnancy test failed to detect that she was already pregnant, when they did the Tube-ligation, much to her surprise she was expecting a month later. No more after that.
I know someone whose twin sister had four kids, two sets of twins. So the sister had her tubes tied. Lo and behold, a year later she was preggers with another set of twins.
Vasectomies can heal after the ten year mark.