u/Era5u5

I've had two pairs of rechargeable AA batteries(different brands) I've used for a while now. The first pair being really old and the 2nd pair being fairly new. Although recently I've noticed that I couldn't use the newer pair, when I put it in devices they wouldn't work. I've checked on a nitecore charger and realised that one of the batteries weren't even registering anymore. I went to my older pair as a backup and saw that the same exact thing was also happening. One of them was dead, at least according to the charger. It confused me because the older pair was sitting in a box for a while. What can cause one of the batteries to suddenly stop registering? In my case one of each from the pairs.