The cause is one of the followings. 

1

If the door is opened during locking operation, the thumb-turn and cylinder might become stiff. 

This is due to the safeguard clutch function which activates during operation to protect the parts when receiving the load, not a malfunction.

The thumb-turn and cylinder will return to normal after being operated by electric function.  

2

The lock (dead bolt) might be in contact with the strike. 
Please adjust the position of strike.
How to adjust the strike（ Door ／ Sliding door ）

3

There might be an abnormality in door's power source.

【When using batteries】
Please reinsert batteries in correct manner or change the batteries.
How to change batteries

【When using AC adapter】
Please make sure that the AC adapter is connected with the power outlet.

【When using controller unit】
Please make sure that controller unit's power is turned ON.