Probably your nib is clogged.
Rapidograph nibs are a hollow tube with a weighted needle running through the tube. Ink can dry around the needle keeping it from moving inside the tube and blocking the ink flow. If you shake the pen gently and don't hear the weight clicking, that's likely the problem.
Also, sometimes the needle becomes bent and jams inside the tube - that keeps it from moving as well.
Thirdly, ink can block the breathing hole located in the nib assembly (the nib assembly screws into the retaining ring, which attaches it to the handle. The ink reservoir attaches to the nib assembly). The nib could be fine but if the hole is blocked, ink won't flow.
Clean the nib, reservoir and retaining ring by removing them from the barrel. Remove the inner nib assembly.but do NOT remove the cover that contains the needle (that's how needles get bent... once bent they cannot be used - the nib must be replaced!) Immerse the parts in methyl hydrate or anhydrous alcohol (alcohol without any water in it... gas line antifreeze works great!) Place the pieces & alcohol in a small container (like a film cannister) and cover the nib completely with liquid. Give it an occasional gentle shake, then leave it overnight. Rinse it after with clear water and that should fix it.
Don't use India Ink in a Rapidograph - it contains shellac which clings to the internal metal nib parts & is almost impossible to remove. Hope that does the trick!
Edit: tapping the pen nib on paper can bend the needle or the nib, and is not recommended. Instead, shake the pen horizontally to get the needle moving, or draw on your finger or palm to get it going.