I did find that the thermometer I used most last summer with the hygrometer is off by quite a bit. But I did a test on the others and they all read the same on each test.I really hope we can help you!
There are a bunch of very good hatch a holics on this thread.
Did the chicks that hatched hatch on time?
I just finished hosting the NYD hatch a long and you would be amazed at how many had multiple thermometer that were way wrong. Most thermometers are not accurate enough--They can be 5 degrees off, both up and down. Safe incubation is 99.5, one degree above and below so 98.5 to 100.5 is ideal. Incubator warehouse type thermometers are better but the Brinsea spot check is the gold standard.
@ChickenCanoe has a link to thermometers that are just as good so maybe he will jump in here and help.
I know most focus on humidity, but Temperature is actually the leading cause of Shrink wrap(Heat is lost when you open the incubator too). Too low of a temperature will make the chick hatch slower. Shrink wrap comes from the shell drying out and if the chick sits too long the membrane will dry not matter what humidity used. That and it will be weak from not developing correctly and will not have enough energy to break out.
So I am hoping I will get better results with these. Some of the chicks did hatch on time but most did not hatch. Out of 12 I only had 4 to hatch. Or out of 6 or 9 I would get 2 or 3 to hatch. I tried like crazy to get my styrofoam bator to 99.5 but it either goes from 98 to 100? I don't know what to do about that. I looked all over for thermometers and only found a few 2 from walmart and two from pet smart and I bought them all!
So if they don't hatch on time then what Ron?