Saw an interesting You Tube on checking the ink flow through a nib to see if it was blocked or if there were other problems.
Leaving the nib attached to the converter, filled with water, you press tissue paper to the back of the nib assembly and you should be able to see a steady flow of bubbles into the converter.
The other was blowing into the back of the converter, again filled with water to see if there were bubbles in the water. This was bad as the back of the converter should be air tight.