Ni-Cd rechargeable batteries have memory. Try a few cycles of discharging them to zero volts then charging, in order to remove the unwanted memorized discharge level.
Another common mode to fail is they grow dendrites internally and short circuit themselves. This fail mode can be removed by braking the dendrites with a very high current pass through the battery. It works but soon the dendrites will grow again, so this kind of recovery is not very useful.
NiCd technology was replaced long time ago by NiMH or Li based rechargeable batteries, so long ago that most of the times it doesn't worth trying to recover NiCd.