I have cut the led strip right where it says the mark and there are 3 leds that do not turn on
May I know if you cut the light while the power is on? It might lead to the short circuit.
Please reconnect the sync box. After all the connection, please unplug the power supply and re-power it after 10 seconds. Let the device reactivate and recognize the light again.
If it still does not work, please feel free to contact our support email for aftersales service.