I have the same problem it started this past weekend... lost each days work when I had to re-pair it. You do the paring in the striiv app you should see 3 bars on the top left side of the app click on that and the you will see the Menu screen. Click on Pair Device. Just follow the instructions from there. But I have had to repair mine now for the past 3 days.
I have open up a ticket with support just waiting to hear back from them (#8569 will not stay connected)
Here is the info from Support:
I regret to inform you that our engineers have confirmed that the recent Striiv update has caused sync issues with the Play. They are already working on a fix. The earliest that we can get the update out with Apple will be Friday, June 13. In the meantime, if you unpair or de-register your Play, the software will keep track of your Steps, Miles, Calories Burned and Minutes of Activity. We acknowledge that this is an incredible inconvenience and thank you for your patience.
Striiv Customer Care
I still have not heard back from support as of today