Wait through day 19: late hatches to day 19–20 are within normal range. Then candle before you clear anything, and diagnose using your temperature, humidity, and egg-age records.
First, the reassurance that's also a fact: a quiet incubator on day 18 is not a failed hatch. Coturnix hatches run day 16 to day 20, and a cool-running incubator pushes everything late. Do not clear the machine on a schedule. Clear it on evidence.
The decision sequence, in order
Day 18–19, nothing: wait. Genuinely. Warm, humid, closed.
Day 19–20, still nothing: candle in place — quickly, without unloading the incubator. Mostly-dark eggs with a clear air cell may still contain live, slow chicks; look for movement, or the shadow of an internal pip in the air cell. Any life or any doubt: back to waiting, check again in 24 hours.
Day 20+, candling shows no movement anywhere: the hatch is over. Before clearing, do the diagnosis below. The eggs are your data, and round two starts smarter than round one.
Pipped but stopped: the hardest call. A pip with no progress for a few hours is a chick resting: normal, leave it. A pip with no progress for 12–24 hours, especially with a visibly dried membrane, is the one narrow case where careful assistance is sometimes defensible, and even then it's a last resort. Opening the incubator to help one chick risks every other egg in there, chicks helped out early can bleed from unabsorbed vessels, and a chick that couldn't hatch often couldn't for a reason. If you must: warm hands, moisten the membrane, chip only shell over the air-cell end, stop at any sign of blood, and accept the odds.
The post-mortem: four causes explain most failures
1. Temperature — the first suspect, always. Whole-hatch failure or a hatch that ran very early/very late points at the thermometer before anything else. Built-in sensors reading a degree or two off is the most commonly reported equipment problem in home incubation. Fix: an independent calibrated thermometer at egg height before the next set, and see our incubator guide on which machines hold temperature.
2. Egg age and handling. Hatchability declines with storage. Extension guidance says to set within about ten days; research on cool-stored Coturnix eggs found hatch rates holding through roughly the first three weeks, then dropping steeply. Eggs stored warm, washed (strips the protective bloom), or shipped (air-cell damage) all underperform. If your set mixed fresh local eggs and shipped ones, tally results separately; the pattern is usually loud.
3. Humidity. Diagnosable from the failures themselves. Fully formed chicks, dead in shell, never pipped or internally pipped only → commonly linked to humidity/ventilation problems late. Shrink-wrapped chicks (membrane tight and dry around them) → humidity crashed during hatch, usually via lid-opening. Drowned-looking chicks or sticky, wet hatches → too humid throughout, air cell too small. Cross-check against your candling notes on air-cell size (what those should have looked like).
4. Fertility — the cause that was never yours. Eggs that candled clear at day 7 were never fertile: that's a breeder-flock question (ratio, age, lighting), not an incubation one. A high clear rate from purchased eggs is worth a polite conversation with the seller.
Set yourself up to diagnose next time
The keepers who improve are the ones with records. Log per hatch: egg source and age, storage conditions, set date, thermometer readings (independent, not display), humidity range, candling notes at 7 and 14, first pip, last hatch, and counts — set / clear at 7 / quitters at 14 / pipped / hatched. Ten minutes of notes converts a bad hatch from a mystery into a correction.
And a benchmark to calibrate against: extension benchmarks for commercial operations run 75–85% of eggs set. A first home hatch of 50–60% is not failure; it's a starting point with obvious upgrades available.
FAQ
Can I open the incubator to candle on day 19?
In place and fast, yes. At day 19+ the information usually outweighs the humidity cost, since you're deciding whether the hatch is over.
All 24 eggs were clear at day 7. Incubator's fault?
No — clears were never fertile. The incubator can kill embryos; it can't create them. Look at the breeding flock or the seller.
Should I do an eggtopsy?
If you can stomach it, opening failed eggs (outside, they're unpleasant) tells you when development stopped — early quitter, mid, or fully formed — which maps directly onto the four causes above.