You're not failing! If 50% of incubated eggs hatch, you're doing fantastic! It's a learned skill (for humans and for non-broody ducks), and it isn't easy.
There are a few things to keep in mind:
1. Not all incubators are created equal
2. Putting fertilized duck eggs of different ages in an incubator can mess up the humidity and the CO2 levels - that's why some people buy fertilized duck eggs from hatcheries. The age problem is solved, but the hatch percentage is still pretty low - maybe 70% is a good number?
3. Incubating duck eggs is hard.
I've got to go do stuff (I'm taking a well deserved break, but it won't last forever ;) ), but I'll check on this thread later.
Do you have Storey's Guide to Raising Ducks? I feel like I'm shilling for this book... . Anyway, if I recall correctly, there's a great section on duck egg incubation. I'll see if I can find it later if you don't have it.
Don't give up! This batch might not make it, but that's part of the learning process. Don't give up!
Apologies for typos and poor grammar - not proofreading...