Start eliminating your possible causes of poor incubation. First make sure all your equipment is sterile. Clean it really well and spray with 10% bleach and let it air for at least 10 minutes, then rinse with hot water and allow it totally air dry. Clean everything you use around your incubators and everything you tend to touch while handling your incubator, like the door knob for the room, whatever, so you don't recontaminate it as soon as your done.
Get fresh eggs from a near source who has a known hatch rate and pick them up yoursef, set them within a week of lay. Keep the temperature and humidity right for storage. Let them warm up gradually to room temperature before set, don't put cold eggs in a warm bator or they will sweat at first.
Have your incubator up and running for a couple days before set and check the accuracy of the temp and humidity by at least 2 ways. Best stuff I got for the job was from a greenhouse supply outfit as it's built to take the humidity. The digitals you can grab at Home Depot and the like are unreliable once the humidity gets high. Dry and wet blub readings are best if you can find some.
Candling is tough, you need to look at many pictures on many sites before you have the idea of what your looking at. If you can get white shelled eggs for the first while. Way easy compared to brown.
Sorry things are not working out for you. I know it's tough, I have had eggs shipped long distances and out of 3 dozen gotten one chick, or none, then 10 next time. Shipping is really hard, but to have it work (and sometimes it's the only way to get a breed) you have to have perfected your hatching. The eggs are weakend by the shipping so they need PERFECT hatching conditions. Work out your hatching till you can match or exceed the rate someone local can get with those eggs. If you can't compare keep trying till you get 75% anyway if they layer flock seems healthy.
By adding in shipping your never going to figure out the issue, as there are too many variables. As you have done with this hatch break-out is the only way to diagnose the problems. Look at the hatch diagnostic charts to find the probable cause for the failure.
Hope that helps.