I’m one of those people Amy is talking about. (That’s OK, I talk about her sometimes) Never trust any thermometer or hygrometer until you verify it is working correctly. I’ve also had bad experiences.
Still air incubators are a bit tricky, where you take the temperature is very important. Hot air rises. In a forced air the fan stirs the air up but you can get quite a bit of difference in a still air based on elevation. Try it. Take the temperature at different levels just to see how much difference it can make.
A whole lot of chicks are hatched in still air incubators, but sometimes there is a learning curve with any incubator.
Did you open the unhatched eggs to see if you could determine when they died and what stage they were in? There are a lot of different things that can kill a chick in the egg. You need some help determining what that might be. If you don’t open some eggs you don’t have much to go by.
In general if an egg doesn’t develop or dies in the first week it probably has something to do with what happened before incubation started. If it dies in the last week it’s normally something to do with incubation.
Temperature and humidity are important, but there is a range of both where you should get some eggs to hatch. But storing them too hot or cold or shaking them up before incubation starts can cause really bad hatches. Incubating them pointy end up can make it really hard for the chick to find the air cell for internal pip. There are a lot of other things besides temperature or humidity that can cause problems.
Good luck on solving it.