Solution :

1. if you log out from the application then you need to scan the smart scale again. Check whether your smart scale is paired with your device or not. Kindly go to settings > scan devices 

2. if you are using the latest mobile phone (android pie and above) then for actofit smartscale app select  don't optimize battery option from settings. (settings > Battery > select actofit smartscale > select Don't optimize battery) 

3. set background limit from settings to allow background processes. (go to settings > search background process > set standard limit) 
4. Check Your internet Connection.