My client's pectoral muscles don't get sore from push-ups.

I have been teaching group fitness for many years and I have a student who never gets sore in her chest from doing push-ups. She gets sore in her back and arms, but not chest. She is a young woman who is very lean with not much muscle yet. I have tried every exercise and modification possible to try and improve pectoral strength. She will be joining the military in a month and needs to be able to do several push-ups. Any suggestions? Thanks!