Well, come to think of it, I've had that problem myself in the past, but never gave it much thought to figure out why. So, in thinking about it for a little bit, a couple of things come to mind..........
Are the cars properly weighted? Something just seems to say that if they're too light, they're not always going to work good on re-railers.
How are the wheel gauges and rail gauges? Having everything properly gauged goes a long ways towards avoiding derailment opportunities to start with.
Are the bolster screws slightly loose, in order to give the trucks a more than just rotational play? This would allow either side or front and back of the truck to go up or down a little bit, in order to try and ride back into the rails at the re-railer sections.
Just a few guesses at this point, perhaps others could chime in their thoughts and experiences.