After switching only a few hundred times between the two extruders, the original servo arm was worn, and the screw had loosened slightly. The result was that the servo arm was no longer able to flip between the two extruders, and a multi-material print failed. I printed a new one, and it worked at first, but wore out again.
Should I be using a harder material for a replacement arm? Am I expected to tighten the screw occasionally? A single very large print could have hundreds of swaps, so I'm finding the durability insufficient.