Most probably the contact between the clip and the leaf could be poor so you need to moisten it. Water increases the electrical conductivity.

Also you need to check that the device is switched on and the sensor cable is plugged in the correct socket of the device.

Apart from these technical reasons sometimes the plant does not want to play. The plant enters into an in-depth contact with its surroundings and the people inside the room or its vicinity and perceives our emotions. Lack of interest, aggressiveness or sense of distrust unavoidably affect the harmony of the atmosphere and thus the plant’s behaviour.