My experience is that some fountain pens just doesn't like certain inks, or ink brands regardless of if they are pigmented waterproof or not. If you like waterproof inks, have you tried a different brand? There aren't that many to choose from, but as Elvis Murray wrote, Jetpens have a good guide about waterproof inks. Personally I have tried Platinum Carbon Ink and DeAtramentis Document Ink. The Platinum clogs after a couple of weeks in the pen, the DeAtramentis does not.