Most chargers have several safety parameters built into them. On most hobby-grade chargers you can typically set maximum charging time, maximum capacity etc. The Traxxas chargers likely have these built into them and are not adjustable as they try to be less complicated so you may just be hitting one of these timers. You may be a tenth of a volt off and triggering the timer, sometimes if batteries sit for a bit they become a little "stale" and a few cycles gets them going again. It'd be best if you could take a look at the individual cell voltages which would give a good indication of where the pack is at. If voltages are within a tenth or 2 of each other it's likely the pack is fine, but if you see a cell that is far off of the others it's likely it has a bad cell.