My harmony test didn't work! Anyone else?

Mary
My doctors office called to tell me they got the "results" of my harmony test. The blood they took from me didn't have enough of the baby's DNA in it. So I have to do it again and wait another 10 days. 
Why would this have happened?? Anyone else with the same problem? 
I was 12w4d when the blood was drawn. I am 13w6d now, and I had it redrawn today.