In ideal situations, the S520 can operate up to 6 feet (2 meters) with the receiver in clear sight of the keyboard and mouse.
If you are not getting this distance:
Replace the batteries with a fresh set.
Relocate the USB Receiver to a line of sight or change its positioning.
Move devices such as cell phones, radios, wireless routers, microwaves, and other devices that emit radio waves or could cause radio interference (RF) away from your work area.
To determine if your environment is causing shorter than expected ranges, try using your S520 in a different environment to see if the distance improves.