Once you refill the cartridge and place in back into the printer, it is always helpful to run a printhead cleaning cycle per the instructions in your printer instruction manual. If running the cleaning cycle 3 to 5 times does not improve your print results, the most likely cause is that ink is not feeding properly from the cartridge into the printhead. It's also possible that the cartridge is overfilled and ink droplets are not able to break free from the printhead to reach the paper. If you remove the cartridge from the printer and squeeze gently, you should see liquid ink at the catridge ink outlet. If the ink drips from the cartridge with a gentle squeeze, the cartridge may be overfilled. If no liquid ink is visible, the cartridge may be dry or old.

Note: The above instructions do not apply to old Canon cartridges with built-in printheads.

Possible causes of this are overfilling, a fill-hole seal that is not airtight, or the sponge in the cartridge is beginning to dry up and won't absorb as much ink. In the case of overfilling, extract some ink from the cartridge. Ensure that your fill-hole seal is airtight. It may be necessary to reseal the filling hole where you injected the ink. Many cartridges have sponges inside. Make sure you penetrate the sponge with your needle, push down to the bottom, then slowly move upwards as you inject the ink. Inserting your needle in different directions in the sponge may also help it to regain some of its porosity. Reseal the filling hole where you injected the ink.

Use a pair of pliers and twist the cap up to remove it from the cartridge. The cap should pop off very easily.