There have been no studies that I know of that show Cryptocaryon/ich can develop a resistance to copper. However, the term shows up a LOT on the hobbyist forums. The reason it seems, isn't that the ich has developed resistance, but rather, there was some some flaw/misunderstanding in the treatment method.
In your case, I think the dose is fine (but you could go to a full dose of 2.5), and in using a Hanna checker, you can avoid mismeasurement. You are only at 5 days at a full dose? it takes at least 3 days just to level off the parasite load, until then, it often increases. By now, the fish may still look sick, but the number of spots should not be increasing. It will take 7 to 10 days for the spots to go away, and then a bit longer in some cases for the fish's skin to heal up.
A lot of this has to do with how severe the infection got before you reached a full dose. Copper Power is very safe, but it also very mild and slow to act. In some cases, it isn't started soon enough, and fish can die from ich, despite being under treatment.
If you post a short video, I can possible help assess their current condition.
Jay