newageofpower The reason that the shield isn't being powered is that the crew that are assigned to get power for the shield are reassigned to get power for the missile factories once they are near the reactor.
Also because there originally were crew assigned to get power for the shield the other crew do not get assigned to get power for it.
The reason some of the modules appear to be working fine is that for this problem to occur it requires the timing of the crew getting power for the shield arriving at the reactor to line up with when the factories need power. Whether this happens or not is basically random but if you have enough modules at least one of them is likely to fail in this way.