More often than not, the Band fails to register the 1 and 2 minute post-exercise heart rates. I suppose it is meant to work like this:
Immediately after finishing your run or the last set at the gym, end the exercise from the action button and confirm the ending of the workout. Do not open the Health application on your phone, but let at least two minutes pass so that the Band can capture the difference in heart rates after one and two minutes. If you check the app too quickly, the Band will probably move the data on the workout directly to the app and cloud without the registered change in your heart rate.
As said, it works for me only occasionally, although it would be a good measurement of one's physical recovery abilities.