All those 6 sets of batteries were working fine till about a week ago
NiMh batteries are pretty reliable so I am thinking you might have some problem with the charger. What's the voltage reading on the batteries? 4 NiMH should be around 5.2-5.3 V when fully charged and should be around 5.0 V during the normal discharge cycle.
You can make a makeshift charger with a regular power adapter (anything greater than 6V) with a small resistor for temporary use. This way you can test whether the problem is with the charger or the batteries. Good luck.