The Nintendo Switch was released earlier this month and there have been reports that the left joystick (called Joy-Con) can suffer from connectivity issues. There is a way to fix it, but it would require you to crack open the Joy-Con and tinker with bits inside. If you're confident with your DIY and soldering skills, read on.
Some Nintendo Switch owners have noted that they are experiencing sync issues with left Joy-Con controller, which uses a Bluetooth connection, when they're playing in TV mode. According to Kotaku:
The issue appears to occur while playing in TV mode and when the controller’s signal is interrupted, such as when another person stands between the Joy-Con and the Switch. This results in the left Joy-Con controller dropping connection or becoming unresponsive.
The antenna on the left Joy-Con is built into the board. Over at YouTube channel Spawn Wave, they opened up the Joy-Con to take a look inside. It appears the signal may be blocked by other components in the joystick.
Spawn Wave ended up adding a cable in to extend the antenna, moving it away from the metal component that is interfering with the signal. While this seems to have fixed the problem, Spawn Wave did note that this was an experiment and does not recommend others from doing this. But if you're a seasoned DIYer and don't mind cracking open your Joy-Con (potentially destroying the warranty), you can give this solution a go.