I had Mason sleeping through the 18 weeks u/s till they made me drink some cold water to try and make him move over so they could see something better.
Caleb was sound asleep with his arms wrapped around his legs and his feet over his face during his 3d scan at 26 weeks. It was very cute.
I wouldn't be too concerned. It could just be unfortunate that the ultrasound coincided with bubbas quiet time. I joked that maybe they should do ultrasounds at night time as mine were always super active when I was trying to relax and watch a bit of teev.