As with any critter, sometimes bad things happen. I've never had a chick die until this year. It was about 7-8 days old, and was growing just like its siblings. Then one morning, I found it barely alive. The mother hen had covered it in shavings and hay. I never did figure out what was wrong.
All in all, however, it sounds like things are going well. BigWRanch is right, though - the next phase you really have to worry about is as they approach laying age. The first few eggs can be a challenge for some pulletts, and you need to be on the lookout for egg binding, low calcium levels, etc.
One spring I had a girl who started laying early (about 18 weeks), and she had quite the troubles - her first three eggs were doubles that were fused at the ends (totalling about 5" long - !). I had to help her get each one out. I had started to think I was going to have to help her for the rest of her life, but happily she then started to lay normally.
