Hello Andy,
If touch-ups aren't working you have to drop back in grit size - with luck 4000 will do it, then you can work back up the grits again. It's best to find out what is wrong though - a strong loupe, say 20x or 30x helps here. If it has been a long time since the last honing session, the bevel might be a bit rounded from stropping and need re-setting.
The C12k is a great stone, but too slow for me - OK if you have a lot of time on your hands. For a good value-for-money hone you should have a look at the Naniwa superhones - the 10k is a great finsher, as is the 12k. If you want to refine the edge a bit more some chrome oxide on a bench strop is hard to beat! Your bench strop can be as fancy as you like, or just a big hunk of balsa from the model shop.
Regards,
Neil