When we refill an ink cartridge by syringe, it starts to spread out from the edge of the refill needle. Due to the existing air inside the sponge, ink cannot follow a regular path. Later on, as we start to print with the cartridge, we come up with another problem. Because air is lighter than the ink, it situates under ink molecules. Therefore, it blocks the ink and situates between the nozzle plate and sponge. This is the reason why a cartridge stops printing after several print outs.
Furthermore, due to the existing air, the ink starts to go over the sponge. And this results in unbalanced air pressure inside the cartridge and leads to ink dripping eventually.