I have a knitting question.

I made a cable scarf that I finished last year, but the outside edges curl inward all the way down the scarf.
I just recently learned about blocking, and tried to steam it flat, while pinning it down. I steamed it with an iron, and when that didn’t work, I sprayed it with a little bit of starch. I don’t even understand why it didn’t work, and if anyone can help me out, it would be much appreciated.