You really need to post more details so people can help you. How are you raising them? By "lost" do you mean you found them dead one morning or that they wandered off/were taken? And so on. The more detail you can provide, the more likely people will be able to jump in with ideas.
I feed them 27% Poultry kent feed, they did not show any signs of being sick, and I could not find anything wrong with them when they were dead. they were close to the heat lamp when I found them dead. they are Runners.