BQ24721 No Ack after send command

Hi  All 

Now Our customer using  BQ27421 , but sometime  "No Ack"  after send command  factory shipping Test.

Customer  checking   Device ID all  MP set at Factory line.

But several set did not response ACK  ( No Ack) 

【　Question　】

1)  Could you tell us this  situation ?   Why No Ack ??  

　　When this problem occurs, what kind of thing is considered?

  　 For Instance 

      Device Busy ??   Signal Level is No good ??   Other Reason ??  　 Do you have anything?? 

Customer already made  more ten of thousands for MP. 

This  " No ACK "issues appear for   several sets .

Regard 

March Jasper  ( Sugimoto  / Japan Disty )