Health data keeps disappearing without reason
Give it an hour then open it again.
For some reason, it can take a long time to catch up. 

Don't try to force it - I did that once in the early days &amp; lost about 3 weeks of data - which replaced itself with a marathon 79,000 steps in one day, then 20 days of no info.

I've noticed it is a lot better since iOS 8.3, in fact, I'd forgotten it used to do it until your post reminded me.