In your case, since you are using copper in the display tank, you are in effect turning it into a hospital tank.
The life cycle of this parasite is very important to understand. The stage where the parasite is attached to a fish is called a trophont. The trophont will spend three to seven days (depending on temperature) feeding on the fish and that is what you see symptomatically when you see "salt sprinkled on the fish". But, and this is a big but, if the trophont is in the gills, you will not see it. After that, the trophont virtually always leaves the fish, always at night, and becomes what is called a protomont. (In the initial stages of infection, the number of trophonts is relatively low unless the fish is introduced into a tank that has been breeding cryptocaryon for a long time; this means it is very easy to miss visual symptoms if any). This protomont travels to the substrate and begins to crawl around for usually two to eight hours, but it could go for as long as eighteen hours after it leaves it's fish host. Once the protomont attaches to a surface, it begins to encyst and is now called a tomont. Division inside the cyst into hundreds of daughter parasites, called tomites, begins shortly thereafter. This noninfectious stage can last anywhere from three to twenty-eight days (hence the three day transfer protocol in tank transfer) . During this extended period, the parasite cyst is lying in wait for a host. After this period, the tomites hatch and begin swimming around, looking for a fish host; the tomite stage is the life cycle stage that is vulnerable to copper treatment. Since fish in aquaria tend to sleep in the same location, it is very likely that tomites can reinfect the fish at night. This is why it is critical to test copper just before lights go out since tomites are the life cycle stage that can be eradicated via copper.
Note, that since you probably have live rock and substrate maintaining the proper therapeutic level of copper will be difficult although not impossible. At this point, the life cycle phase that is vulnerable to copper, they are called theronts, and they must find a host within twenty-four hours or die. They prefer to seek out the skin and gill tissue, then transform into trophonts, and begin the process all over again. What this means is that when your tank is infected, you can actually see symptoms during a very small part of the life cycle, and it is why your tank is infected even though your fish can be resistant. It will also explain why visual and/or behavioral symptoms come and go.