There are many reasons developed eggs do not hatch but if they were from your own birds the first thing to check is nutrition. After that I make sure my 'bator is pretty clean. Not Clorox clean because hens don't use cleaners but a good water and scrub with maybe some Oxine, if it is real nasty smelling.
I just had a hatch and of all 36 eggs only 2 did not hatch. All mine except for the 3 duck eggs. Those 3 hatched but only one lived. Again, not my eggs or birds on those 3. Supposedly they are fed just like mine, Williams mash and some whole grains. BUT, mine are out eating fresh grass and bugs most of the time and these ducks do not get that.
I open my bators constantly. I do not believe lock down is important. I move mine to a hatcher on day 18 and that lid is opened numerous times each day.
Consider where your incubator is located in your house. They run and do the best in a central part of the home with very little air change. Mine actually are in the center of my house with us walking past them constantly. However, I am sure it stays the same temp there because my wall thermometer is above the 'bator. Not my heat/air switch but just a temp read out.
I used to have my 'bator next to a south wall but got a bad hatch once and tried another spot. Haven't had one since.
Try adding some Rooster Booster or some other type of vitamin/mineral supplement to their food or water. I add Dawe's A-D-E a few days a week, ACV/garlic a few other days and plain water one day a week. I get very good hatching results from my own breeder birds. I'd have to go back into my book for this year but just off the top of my head I want to say I am getting 98% hatch rate.
It really does not sound like a 'bator problem.
Oh, and Susanne, I hate to hear about you losing a 'bator and all those precious eggs! I would have cried!