Next time you might want to try a higher protein grower. Meat birds need a lot more protein than the slower growing 'regular' chickens.
I am currently using 21% protein broiler grower from Agrimaster and am quite happy with it.
What kind of feed are you giving them? What protein percentage? Can you cut open this last one that died? Is there lots of fluid in the body cavity? Are the intestines filled with blood?