I have a 6+ . on 8/30 it stopped recording data points for the steps.It is still tracking and sharing with other apps but the graph is just dotted line going from the 29th to the current steps. I have rebooted and taken the steps off the dashboard and readded. I have tried adding the steps under Add Data point and it puts it for a second but reverts back. I thought about deleting the app and re-adding but don't want to loose my stats.
You can't delete the app since it's stock. Only suggestion I've seen after doing a quick google search is to reset or power the phone off for a few minutes, then turn it back on. Apparently there's a glitch. Perhaps it will be corrected in iOS 9? I don't use the app that often so I don't know if mine has experienced the issue.