Brooding hen sitting on eggs, chicks break eggshell then are dead

I have a Rhode Island Red hen that is sitting on her eggs, most recently 5 out of 6 chicks broke their shell then are dead by the time my husband finds them. He checks on them in the early morning, mid afternoon, and dusk. We live in San Antonio in low 100s heat. She is in a small chicken house and it is very hot in there. I was wondering if they were getting overheated! We have had several other batches and have had about 50% survival rate. Any ideas?? We are very new at this.