20wk ultrasound blues

Chelsea
I was so excited when my midwife told me that I could make my 20wk ultrasound appointment for 3 weeks from today (I'll be 19wk6d) only to get to the appointment desk and be told that the ultrasound people won't let me have the ultrasound that early..... it's one day early - one day! .... now I have to wait until the Monday after. I know if doesn't seem much longer, but my hubby and I did the harmony prenatal test only for the gender result to come back inconclusive twice - once at 10w2d and again at 13w3d. I've been worried. 
I wanted to rant, but I also wanted to know if there is a legitimate reason why the ultrasound people at the hospital won't do a 20wk scan a day early. The only response I got from the desk was "yeah, they are really strict about it"
Thanks in advance for any help!