LTC4162EUFD-LAD charger (12V/2.5 A) problem

Hello,

I am developping a charging board for lithium battery (2 cells) based on LTC4162EUFD-LAD with a charger 12V/2.5A. I made some samples and all working fine . Now we produce the pre-release series and one problem comes out ( identical components and PCB layout). If we try to charge the battery using the charger 12V/2.5 A the LTC4162 fails battery detection ( repeat battery detection every 30 secods as datasheet describes). Instead with a charger 15V/2A all works fine. Do you have some suggestions? I mean where  should I check to verify why battery detection fail.

Thank you.

Gaetano