Extract from sharpening made easy
"Diamond stones will sharpen a ceramic knife, but you must remove all scratches caused by the diamonds. Scratches act as stress risers and can cause the brittle ceramic blade to fracture.
Silicon carbide wheels or stones can be used to sharpen ceramic knives, which are made of relatively softer aluminum oxide. Since paper wheels use silicon carbide abrasive, they too can sharpen ceramic knives. SC wheels can also remove the scratches from sharpening with diamonds.
Ceramic blades will not raise a burr. You have to use the other tests to determine if you have created a new edge."
I suspect you've probably gone through the grades of diamond too quickly. You probably need to inspect the edge with a jewellers loupe or linen tester to see when to move up to the next grit, or use a magic marker (use one that can wash off) on the edge to see how you're going.