Considering he was a medicinal scientist (before retirement) used to handling lab samples in controlled conditions, i cannot accept he contaminated his sample.
It's not necessarily that the sample was contaminated (unless Ancestry explicitly told you that). Usually it fails because they were unable to extract sufficient DNA from the sample to run through the processing routines. It can be a particular problem for older subjects, as their saliva often contains less DNA and/or they may have difficulty producing sufficient saliva.
I failed three consecutive tests on Ancestry and the first attempt on LivingDNA. I finally had success with Ancestry after filling the sample tube to a few millimetres above the indicated level, and also rubbing the inside of my cheeks with a covid type mouth swab and leaving it to stand in the test tube for 30 minutes before removing and sealing the sample. My brother failed his first attempt with Ancestry, but the second went through after I advised hime to try the same routine.
