VivoSmart 5 stopped detecting my activity when I go for power walks.

What exactly do you call a "power walk"? Are you using hiking poles? If so, try starting the walk without the poles for a few first minutes. Also double-check that the auto-activity-start option is still enabled.

Yes, I just started using the poles. I'll try that tomorrow and let you know.

... and if you carry the poles (without using them), make sure you carry them in the other hand than the one with the watch. The arm wearing the watch must be freely swinging in order to detect the walk, and in order to count the steps accurately. BTW, using the poles usually reduces the number of detected steps, exactly for this reason. The resulting count may be easily halved, when using hiking poles (though it depends on the walking style, the terrain, and on other factors).

vcallinan said:

Which is a shame because in winter walking, especially in a freeze-'n-thaw area, poles are a good safety measure. 

Not sure who should be ashamed that the accelerometer of the watch cannot detect the movement when you use hiking poles. If the arm with the watch cannot swing naturally, the watch has no chance to detect the walking pattern. There is nothing to be ashamed for. If you really need to use the poles, simply start the activity manually. You can also try wearing the watch in the pocket, in such case. There is a better chance that the watch detects the step pattern when in the pocket, than when using hiking poles.

Thanks for the tips!