We've noticed the calibration tool on the V2 SDK has had some issues with current OS versions and will show the LMC as disconnected when you click on calibrate device in the Leap Motion Control Panel.
As legacy piece of software we aren't maintaining V2 any longer however there is a workaround available for this issue. If you are using Windows 10 you can uninstall the V2 SDK, installing V4 and calibrate, then reinstall V2. The calibration data is stored on the device hardware so it'll carry over.
If you are using our later V4 software and seeing this issue please send a screenshot to firstname.lastname@example.org and we will have a closer look.