If rust has arrived with a particular set of plants, it may be that there is a delay in time from when active spores would have spread, cross-infected, and show up on newer plants? By then, weather conditions may not favor as much spread, even if all arrivals had rust? Just some thoughts. I saw that happen with my carnations ... first-arriving plants were severely hit (and died, though I'll keep them in ground in case they can bounce back) while later arrivals to the garden were less impacted after being planted once our short wet season had passed.
