If its unsoldering at the bullet, the battery port must be dirty and not allowing good contact or the bullets are squished in. Doesn't have to be thrashed, just needs to be a little loose in the port to arc and create enough heat to melt solder.
Try sticking a pin or tooth pick or exacto or something down the center to squish them back out or better, for $3 if they have them at the track, replace.