FYI, I had a pair of clowns in quarrantine for about a month and last week I realized that I had neglected to change their filter media like I know I should and they developed ick. I immediately put them in a hypo tank and within 48 hours they were ick free. This is not news to me as I already know it works but given the topic of discussion, I thought you all might benefit from the info.
I have probably only used copper three times in as many years to treat fish. Hypo works pretty much every time for me. I can't help but ponder if those who are unsuccessful with this method of treatment might be either lowering it too slowly or else might be not be bringing the SG down far enough; whether that be from fear of the process or inaccurate swing arm hydrometers giving them false readings. The only other thing I can think of that might not allow it to work would be mis-identification of the pathogen.