When the cells sit for too long without any cycling, the internal resistance skyrockets. Most quality chargers will not be able to recognize the pack once it gets too high. Luckily this can often be (somewhat) reversed with a forced charge. Another option I did not mention was a wall type trickle charger, such as what comes with many RTR vehicles, is basically the same as a very low setting on your charger with no peak detection. The difference is that these wall type chargers do not recognize a problem and will charge anyways. This is whats needed to "wake up" most dead packs. If you have one lying around, anything from 0.1A to 0.5A, you can hook it up and leave it for several hours, then discharge it hard, then your main charger should pick it up.