Recently a friend had a bad first hatch in a new incubator, and asked my advice. I recommended running the incubator without eggs and using a digital thermometer with a corded probe to record the temp in various places where the eggs would sit. A min/max capability is very useful as well. Here is one I bought that seems accurate enough for me: https://www.amazon.com/gp/product/B07L88FYNW
Draw a diagram of the egg area and write down the min/max in each location. Check areas until you get a pretty comprehensive picture of the temperature gradient. So, how much is too much? I cabinet incubators, I frequently see a temp gradient of a whole degree between the top and bottom shelf. The newest eggs should go on the top and the older eggs moved down to take advantage of the internal heat the growing chicks produce. However, in a tabletop incubator I think 1 degree across the area is too much.
If there is a large gradient, that would indicate that the air flow is not sufficient, or not properly designed. That can be very hard to fix. Clean the fans if the incubator is not new, and check that they are running.
If the temps are pretty consistent across the entire egg area, what’s next? What is the temp you recorded ? Forced air incubators are generally run at 99.5 to 100.0 degrees F. If you are in that range and still had a poor hatch, check the accuracy of the thermometers against a child’s digital fever thermometer. Put it into the incubator, where the thermometer probe is located. Wait about 5 minutes to get the thermometer up to temp, then open the incubator, push the button and close it quickly. The thermometer will record the temp as long as it is increasing. When it finally beeps, remove it and compare it to the thermometer you are testing. If this is the incubator’s thermostat, and it is not matching, try to adjust the temperature “offset” or else adjust your incubator set temp to correct for the difference. Wait a while and retest until you get it in the range of 99.5 to 100.
That’s all I have to say about temps. Humidity is also important, we will discuss that in the next entry.