Hi,
How old are the batteries?
The phone could say that they are charging, as long as the voltage/current from the charger is the correct value.
If the batteries are old they could be failing to hold their charge.
Even rechargeable batteries have a life span. (the average life is around 500 charge / discharge cycles)
Try replacing the 2 batteries with new ones, in just the one phone to see if this resolves the problem. (Allow them to fully charge before first use) They are not that expensive.
The batteries that are required are Ni-MH 1.2V AAA (R03) size.
Here is an image taken from the User guide that shows how to replace the batteries.
(click on image to enlarge for better viewing)
Here is a link to a supplier. It is not a recommendation to use them, it is shown to give you an idea of the cost of the batteries. There are plenty of other suppliers available for you to choose from.
Hi,
Is there enough contact spring tension being placed by the charger connectors onto the phone charge connectors?
With the power disconnected from the charger you may be able to feel the tension in the charger connectors by gently pressing down on them with a wooden toothpick or other similar probe that fits. Check if there is a difference in the spring tension between the chargers that work and those that don't
Are the charger connectors, clean and shiny (this should ensure a low resistance connection)?
Are the phone charge connectors also clean and shiny (for the same reason as above)?
It may be that if there is not enough contact pressure or there is resistance in the connection between the phone and charger that the phone may intermittently not charge.
As you say that they are new, if all else fails, verify the purchase date on the sales receipt or proof of purchase document and if the date is still within the manufacturer's warranty period. then consult the manufacturer's warranty statement in the User Guide as to what to do to make a claim for a manufacturer's warranty repair or replacement.