If you've already followed the 30 day QT copper protocol, I suggest you have a little patience and take a couple more weeks to follow the prazi protocol.
Prazi is only effective against live flukes. The medication causes the live flukes to go into spasms and ultimately detach from the fish and fall to the bottom of the tank. They then starve due to lack of food. If you had flukes in the first place, during the copper treatment they have likely laid eggs either on the fish or on other surfaces in the tank. Those eggs are unaffected by prazi and will ultimately hatch.
It is true that the life of prazi doses diminish over multiple treatments because it feeds bacteria that renders it ineffective. I administer prazi in my QT as soon as I reduce the copper concentration. to between .75 and 1.25 ppm. After 7 days I administer a second dose. After 7 more days I administer a third dose. Then, since the effective life of prazi is very short at that point, I move the fish from QT to DT after 48 hours.
Perhaps I'm overly cautious, but once I move a fish to the DT, I really hate trying to catch it to move it back to QT for something that I might have missed in the first QT treatment.