I have been running into troubles with Samsung Smartthings Water leak and Motion sensors. I could never get them online for more than12/16hours.
I have made some experiences to understand whether the distance was the issue, so I place the sensors 2 meters away from the hub. I have done the reset of both sensors, pair them again and voila, after 8 hours, they went offline again.
So, I thought the problem was the Hub (V3). My next move was changing the Presence Timeout value for 10 min and power off/on the hub. I will try to fine-tune it by decreasing it may be up to 4 min.
No need to install the sensors, they went immediately online and so far for over 24 hours.
If you are having the same kind of problems, you might want to try the above.