An interrupted ink flow is frustrating because the pen often writes perfectly for a few lines first. Resist the urge to squeeze the converter or force the nib. A short, repeatable check usually finds the problem faster.
Start with the easy causes
Check that the nib is fully seated, the converter or cartridge is secure, and the cap has not been left loose. If the pen sat unused, the feed may have dried at the tip. Wipe the nib gently with a clean, damp cloth and test on ordinary paper.
Do not add water or another ink to the reservoir as a shortcut. Pilot’s iroshizuku guide says to wash the nib and section thoroughly when changing colors and warns against mixing inks from different companies.
Separate the pen from the ink
Try the same pen with a known, compatible ink. Then try the suspect ink in a pen that normally writes reliably. One change at a time tells you whether the problem follows the ink or stays with the pen.
If the problem follows the pen, inspect the converter connection and nib alignment. If it follows the ink, check its age, storage, and whether particles or residue are visible. Do not scrape the feed with a sharp object.
Paper can make a good pen feel bad
Highly absorbent paper can make a fine nib feel dry. Coated paper can slow drying and make a flow problem look worse. Test a short line on the paper you use most, then on a neutral writing pad.
When to clean
Flush with clean water according to the pen maker’s instructions. Let the parts dry before refilling. Pigment or heavily saturated ink may need more care, so use the manufacturer’s guidance rather than a universal internet recipe.
For a broader overview of Japanese ink shopping, see Fountain Pen Ink in Tokyo. The best ink is the one your pen can deliver consistently on your actual paper.
The short answer
Check the seal, isolate the ink, test another paper, and clean gently. Change one variable at a time and stop if the nib feels damaged.