The principle of refilling ink for water-based pens is the same as that of needle injection. When making a refill, first fill the ink, then block the tail with that colorless semi-solid substance, then pump away the air inside, and then seal the nib, and a refill is completed. Because of the internal vacuum, atmospheric pressure pushes the ink out of the nib, which explains so much to you that you should be able to think of the reasons why water-based pens are difficult to write. The internal vacuum must be affected. That's because when you fall, the tail of the pen is facing down. Semi-solid things move backwards due to inertia, similar to sucking things with a needle. Naturally, a little air is sucked from the pen tip, and the ink can't be squeezed out anymore. There are solutions, such as sucking the excess air out of the nib with your mouth, which is disgusting, or blowing the ink out temporarily from the tail, but the effect is not very good. After all, the vacuum environment cannot come back. I am a student. I suggest you buy a new one. Not expensive either. Just be careful when using it in the future.