Flashing, and itching is the same thing.
It could be a number of different things wrong.
Best thing would be to find a Koi hobbyist/Farm in the area, and see if someone can scrape and scope for you. That will weed out any parasites, and then you can take it from there.
But ultimately, we need to know more information.. what are your water parameters?
*** now that I look at the pictures of them close up, I did notice that your koi is exhibiting signs of bloat- which is when their scales pop looking like a pinecone.- Bloat is due to 2 things, extreme cold water while being fed, which leads to them not being able to digest food, but the #1 reason is bacterial infection. It will kill your fish. And their is no treatment for it. The only thing you could do is salt to .3% (1 cup solar saly per 100 gallons) and hope for the best.***