If they have a portable u/s machine available at your next appt., at 22 weeks you should be able to see the arms moving no problem.
They would have told you if they had seen or suspected something was abnormal, but I am sure they wouldn't mind if you insisted on another u/s at your next appt.
Yes, I think if something was wrong the doc would have said something. If you can, try to get another one later...ask your doc and let him/her know that you are concerned and they will explain why you couldnt see the baby's hands. Dont stress though, I am sure the doc would have said something if there was anything wrong.