What kind of incubator are you using? Your first move should be to get a guaranteed accurate thermometer or calibrate yours.
I wrote this a few days ago as a succinct guide to incubation.
I think there is a huge learning curve to the point of consistently successful incubation.
Where many go wrong initially, including myself is relying on instruments. Most are wrong. The most important things are precise temperature control, frequent turning, adequate weight loss, using fresh eggs that have been properly collected and stored, enhanced breeder nutrition, incubator fumigation and ventilation.
First of all, one has to start with good fresh eggs packed with enough nutrition to carry the embryos to term. That means good breeder nutrition. Most feeds are adequate for producing eating eggs but not for high hatchability. These numbers may be a little different for ducks but for chickens, nutrient amounts in feed should be 0.5% Methionine, 1% Lysine and 0.75% Cystine, 85 ppm of manganese, 5,000 IU vitamin A, 2500 IU vitamin D, 50 IU vitamin E (the vitamin amounts are per pound). Sufficient riboflavin can prevent curled toes.
When buying shipped eggs, one has no control over breeder nutrition or how old the eggs are before they are shipped. If those numbers aren't on a bag of feed guaranteed analysis tag, you should be able to call the feed company to get the numbers.
Then the eggs have to be properly stored. Cool, humid environment and turned at least once a day. One should try to incubate eggs less than a week old.
A clean disinfected incubator is important. Alcohol works but I've found activated oxine to be best as it kills viruses, bacteria and fungi.
One will never have successful hatches with too low or too high of temperatures. A calibrated or guaranteed accurate thermometer is imperative.
Most people start with one or more cheap thermometers like Accu-rite and reptile/aquarium types - or worse. All thermometers are SUPPOSED to be accurate to ±2°F(1.1C). That isn't close enough for hatching. Add to that the fact that most of those are not even within that standard. I had a thermometer from an incubator company that was accurate at 75F and off by 4 degrees at 99F.
The single move that improved hatching for me was to throw all of them away and get a real thermometer. The best thermometer I've found that won't break the bank
https://www.thermoworks.com/RT301WA It is accurate to ±0.9°F (±0.5°C) off the shelf and calibratable.
Another good one is the Brinsea spot check.
https://www.brinsea.com/p-394-spot-check-digital-incubator-thermometer.aspx
Be sure to get it from Brinsea. Some from resellers like Walmart and Amazon have been defective.
Proper weight loss, controlled by humidity, for all avian species is 12% by lockdown. I have a couple pretty good hygrometers now but I gave up on them for years, opting instead for weighing eggs with a gram scale. That has proven effective and IMO, more accurate than measuring humidity. All eggs are different and need different humidity levels but regardless of species, the percentage of weight loss should be about the same.
Using these guidelines can significantly enhance success.
Bottom line is to never trust the temperature reading on an incubator without verifying.
