The cause is one of the followings. 

1

The smartphone might not be registered. 
Please download the application and register to the door.
How to register

2

Smartphone's Bluetooth might be OFF. 
Please turn ON Bluetooth function on smartphone.

3

The application might not be active right now.
Please activate the application. (Background activity is OK)

4

When putting remote control in trousers' back pocket, the body of the user will act as a wall to interrupt with Bluetooth which results in a weak connection. 
Metallic objects can also interrupt with the Bluetooth connection.
Please secure a close distance between remote control/smartphone and the door before operating.

 

5

The smartphone registration might be deleted by someone else using the application.
Please check if other family members have deleted the registration and register the device again.
How to register

 

6

Touch operation function might be disabled via application.
Please enable the function by opening 「My Entrance」 application then go to 「≡Menu」→「Digital lock setting」→「Touch to lock/unlock」.