I have the same issue and have tested on an outside parking lot stairwell while connected to my S7 edge. Went up 6 flights yesterday and it counted 2. I've only had my Sport for a couple weeks now. On day 1 when I tried it in the house it worked fine and only appeared to miss 1 or 2 flights. since then it seems to not capture the information very well. Additionally, the other day I noticed the auto-walk feature doesn't seem accurate either. Started walking, system registered auto walk....20 minutes later it indicated I had walked for 10 minutes.
Hey, everyone who is having this difficulty, please reach out to 1-800-726-7864. They will be happy to provide you with the information necessary to ensure that your device is properly repaired.
Thanks so much to all of you for reaching out to us about this. Please let us know if you need help with anything else.