Unfortunately TTM isn't always 100% successful either. Studies have shown that ich can encyst on fish scales and likely also bony structures like the scalpels of surgeon fish.
To be really sure you may need to double the total time and number of transfers or do it in combination with other measures like hyposalinity or CP which prevent ich from encysting successfully.
The other issue is that the 72 days fallow recommendation is based on one single observation with one single ich strain. There may very well be ich strains that can lay dormant for longer, so if you want to be on the safe side you might want to extend the fallow period.
In essence, we are dealing with live organisms, so there will never be a 100% security on anything.
Then there is the copper issue - this toxin shouldn't be used anymore since it only makes things worse. It may kill ich but at the same time impair the fish's immune system. So when taken out of the copper the fish will be highly susceptible to new infections of all kind.
The other point of concern is of course that your fish got sick at all - even with ich in a system fish don't have to be sick. Healthy fish will quickly gain a sufficient level of immunity so they don't get sick with ich or die of it, even if it isn't enough to fully prevent infection.
The fact that your fish got sick again can either be attributed to the use of copper or indicate a deeper issue with your tank.
