Hi thanks for your question
Your 16 year old and want to know why didn't you concive from unprotected sex
.well there are many reasons for that ;
No1. Your 16 years old and women in early period of their puberty
don't ovulate (release egg) though they menstruate regularly.
No2 Your boy friend may be having problems like low sperm count
or abnormality in sperms motaiity and morphology.
There are many more reasons which need battery of investigations, but these two are common for your age group
Hope I answered your question