First check whether the pen is blocked or deformed for small impurities.
Some transparent cores can check the water of the pen again and again.
If there is no above, take a piece of paper (the book cover of the book) first, and draw more than 10 seconds or longer on the paper.
Or find a used cigarette filter sponge, use the head of the pen in the filter cotton back to wipe back
Or soak the pen with hot water and shake it out
Or change a good pen that has already run out of "water"
Or in a more troublesome way, at the upper end of the refill, a small hole is added (higher than that of the water surface).