That often happens when either the handset or the cradle (or both) has a problem related to the connector.
The metal contacts in either part are worn, or have oxide on top and the two unit do not get connected. The same handset can connect to a different cradle with for example no oxide problem, and the cradle with oxide on contacts may work with an handset with no contact-related problems.
Answers & Comments
Hi.
That often happens when either the handset or the cradle (or both) has a problem related to the connector.
The metal contacts in either part are worn, or have oxide on top and the two unit do not get connected. The same handset can connect to a different cradle with for example no oxide problem, and the cradle with oxide on contacts may work with an handset with no contact-related problems.
Regards.
If they are six months old the solution is quite simple.
They can be replaced under factory guaranty. Contact the number listed on the owner manual and get the defective units replaced.