It sounds like the screws are stripped out inside. There sadly isn’t much you could realistically do about it. The iPhone 6s is not water resistant in the first place and leaving the screws out definitely would not help. Dust also could get in too. You could replace with a b stock oem case such as the iPhone 6s OEM Rear Case or an unbranded new one such as the iPhone 6s Blank Rear Case but it is not a super easy procedure as seen on the iPhone 6s Rear Case Replacement guide.
