I have an iPhone and the settings are different - but I did set it to not go to sleep. It is my phone. and it happens to a different degree every day. I have tested it several ways, including having my husband's phone on me and comparing our actifit progress (on days when he is not getting enough to post anyway).
His phone always shows way more than mine.
For now, there is nothing I can do about it. A new phone is not in the budget - and I am not sure if I want to stay with a new apple phone. but if I don't, my devices are not working together anymore... So, for right now, I am dealing :)