It might be clogged with mud from the 500? Not sure, but that would be my guess. I think you'll have results using a diamond plate to flatten.
I use a JNS 1K and it cuts like crazy when freshly lapped, but does collect a little swarf along the way, although never so much that it stops cutting effectively. I usually give it a little rub down with a diamond plate between knives.
Hopefully someone smarter than I will have a definitive answer for you.