If ink is not flowing properly from your pen, the ink cartridge should be replaced.
There are two sizes of cartridges: Ink cartridge A refills sizes 0.03, 0.05, and 0.1. Ink cartridge B refills sizes 0.2, 0.25, 0.3, 0.35, 0.5, 0.7, and BS. To put in a new ink cartridge, pull the old cartridge out from the bottom, by using the larger of the two openings on the SP Changer or by simply pulling it out by hand. Next, push your new cartridge, without the lid, up into its place. Download this pdf for a visual explanation.
To replace the nib, use the SP Changer. This product can be used by placing the small opening around the metal indent towards the bottom of the nib and pulling upward. To put your new nib into the multiliner push it into the pen and make sure the wick makes clear contact with the ink reservoir. Download this pdf for a visual explanation.
Avoid frequently pulling the nib in and out, this will cause damage to the wick and result in poor ink flow.
You can achieve the thick/thin line variation when drawing by pushing heavier or lighter. Even though the pen tip is very firm and has no flexibility without breaking, it is still possible to push less or more. The trick to this is have something softer under your paper other than just a firm table surface. Always keep a few sheets of scratch paper under your work, the added depth has a slight give to it so you can push harder without damaging the pen tip. Working in a sketchbook is a great way to get this line variation.
Paper Use and Care
Copic Multiliner inking pens work best on very smooth grained paper. The Copic Multiliner inking pens also work well both with watercolors and on watercolor paper. When working on rough surfaces, caution is recommended while using the finer nib sizes.