. . .has to be pressed quite firmly onto the nipple in the section so just make sure you give it a little extra finger pressure when putting it on. You should be drawing ink up through the nib with the converter already in place. After filling try blotting the nib well with a tissue and/or expressing a couple of drops of ink. This will ensure that the nib isn't oversaturated. The likelihood is that the nib is just too full of ink and so is running ink into the cap.