It's not normal to go dormant that quickly after bloom. All true lilies (including Stargazer) need to replenish food stores in the bulb after blooming for growth next season.
There is not much you can do about the struggling one. It may have bulb rot, or it may just have struggled to produce a bloom and used up all its energy to flower. If when they were flowering, they all seemed to flower with about the same vigor, then I would guess the bulb has rotted and that particular bulb will be dead. (However the others are unaffected.) If the affected plant had noticeable smaller or deformed flowers compare to the others, then I would suspect either the bulb has rotted and dead or the bulb is depleted, but still alive. if it is still alive, next season it will be much smaller and won't be blooming.
BTW, if you are in the subtropics, will it get cold enough for the seasonal change that Stargazers require? I don't think so.
