Having now had the watch for 10 days, I have been observing what it does, and have concluded that it must get confused if you stand up at the turn of the hour. Sometimes it can be OK, sometimes not. I have also discovered that if you have been standing or moving a lot within the hour and it did not register, you don't get a stand warning, and it won't give you credit even if you run around in the last 10 minutes. I have tried standing up and raising my arms, and lowering my arms, but nothing works once it has decided it is not going to credit that hour. I seem to get credit most hours but today it so far missed one that I know I should have had, and yesterday it missed two.
I have also found that the only way you can tell if you have been credited is if you are keeping count, because it is difficult to tell by looking at the bars in the app. You only know for sure you have missed an hour when the hour has passed and there is a grey bar instead of a blue one. It would be nice if there was a clearer indication that you have completed your stand for that hour.
It's all rather frustrating, the activity app was one of the main reasons I wanted the Watch. However I am finding I'm a lot more active as a result of the problems with the stand app!