why didn't the geneticists catch it the first time we got my son tested?

Back when my son was 5 years old, we took him for genetic testing, to hopefully get some answers on what was going on.  But there were no red flags at the time, and there was no diagnosis.  Then we went back years later (when he was 10), at which point they did come back with a Sanfilippo Syndrome (type C) diagnosis.  Why didn't they catch it the first round?  A Duke scientist explains. 

Dr. Sage Arbor (PhD) is a biochemist at Duke University, and he also happens to be the author's former college roommate and close friend.  He may not be a geneticist, but he certainly knows a heck of a lot more about this stuff than I do, which is why I wanted to pick his brain. Sage theorized that the "false negative" test result at the time may have been because of the the specific testing procedure used. I was confused. But he then explained to me that there are two common ways of genetic sequencing that are generally performed during these genetic testing procedures - one is called the "microchip" method (cheaper / doesn't catch everything); the other method is the classical genetic sequencing (more expensive / more accurate).  The main problem with the microchip method, he went on, is that if the test does not happen to include the exact genetic defect that the particular patient has, then the test will miss it - thus, the greater likelihood of a "false negative" test result.  The tests are better today than they were back then, he said, so hopefully the false negatives have dropped over that time as well.  Check out this clip (below) where Sage breaks it all down for us...