Fountain pen ink can look too light even when the bottle is perfectly usable. The paper may absorb the line, the nib may be too fine or dry, or the ink may lose some of its apparent depth as it dries.
The quickest way to find the cause is to change one variable at a time.
Start with a second sheet of paper
Write the same short sentence on your usual notebook and on paper made for fountain pens. Absorbent paper spreads ink into the fibres, which can make a line look both wider and paler. Coated or fountain-pen-friendly paper usually keeps more ink near the surface, so shading and colour variation are easier to see.
Do not compare a fresh line with a dry line. Give both samples the same amount of time, then compare them in the same light.
Check the nib width and flow
A fine or extra-fine nib puts down less ink than a broad nib. A dry-writing pen can make a normally saturated ink appear weak, especially when you write quickly.
Try the same ink in another pen if you have one. If the colour becomes noticeably deeper, the issue is probably the nib or ink flow rather than the bottle. If the pen is also skipping or stopping, follow the troubleshooting order in Why Your Fountain Pen Ink Keeps Stopping.
Avoid pressing harder or spreading the tines yourself. A nib adjustment can create a different problem and is better handled by someone who can inspect the nib.
Separate colour from drying behaviour
Some inks look stronger while the line is still wet and settle into a lighter shade as they dry. Test a swatch and a normal writing sample together. A swatch made with a glass dip pen may look much wetter than the fine line produced by your everyday nib, so use both only as different reference points.
If the line is pale because the paper is drying it quickly, the paper test above will show a larger difference than changing bottles. See Why Fountain Pen Ink Dries Too Fast for a related check of paper, nib, and cap conditions.
Check the bottle and converter
Confirm that the ink has not been accidentally diluted by water left in a poorly rinsed converter or pen. If you recently cleaned the pen, let it dry properly before judging the next fill. A converter that is not drawing a full fill can also make the first writing session inconsistent.
Pilot’s iroshizuku care guide recommends keeping the bottle away from direct sunlight and high heat. Store the bottle as directed, and do not add water or another ink simply to darken a sample you are still diagnosing.
The short answer
Test the ink on fountain-pen-friendly paper, then compare another nib or pen. If the line is still too light under the same conditions, the ink may simply be a pale colour or a poor match for that nib width. Treat the result as a writing preference, not automatically as a defective bottle.