Where did you get your eggs from? Perhaps the flock has something effecting the mortality of your chicks? It is possible. Don't beat yourself up, it happens to everyone at some point. It is disheartening to have losses like this.
The first thing I think of in a case like this is Marek's. I got a number of chicks of different breeds from a very well-known breeder this spring. I had one breed that I lost about 40% of my chicks. I only lost a couple of the other six or so breeds that I got from him (I had about 10-12 of each breed, some I had more). I was stumped because these were healthy chicks at one feeding and 10-12 hours later they were dead. But the odd thing in my case is they were all from the same breed. I know that flock is tested and is supposed to be clean, but you just never know what may cause things like this.
I hope you have better luck in the future. Sorry for your losses.