you might start by measuring the voltage of each cell individually and see if you have one cell that has dropped. mine have usually failed from overheating, either pulling too much or got the cell too hot when soldering and melted the seal/vented the cell, sometimes you notice it to right off, sometimes it takes a while for problems to rear up if you melted the seal. I'm not sure about the open or closed circuit failure so I can't help you there. btw, I learned to solder packs together using cheap surplus cells from phone battery packs, burned plenty of em up but now I can solder the good cells together with out problems (those cheap batteries are good for low amp draws in small planes, ussually less than 50 cents a cell) I'm sure you will get plenty of usefull info here but if not, hope I helped.

Jason