I wanted to change inks and so followed the directions for disassembling and cleaning the piston, and everything seemed alright. I cleaned, then reassembled my pen, reapplied some silicone grease around the piston, and tried to fill the pen... and it will draw only a few drops of ink! What am I doing wrong? What can I do to get a full fill of ink? Normally I'd estimate a full fill at better than 2ml of ink...