My son is 17 years old a full time student and does not work . So why didn’t I get the credit for him as well as my 13 full time student daughter

Because he is 17.  To get the larger credit, like you received for your daughter, your child must be under 17.  You should still receive a $500 credit for him though.  Child Tax Credit

This comes as a big surprise to many parents the year their child turns 17. A child over age 16 no longer qualifies for the Child Tax credit (CTC). Although a child can still be a student dependent through age 23, and a qualifying child for EIC,  the Child Tax Credit expires the year they turn 17 and you no longer get the $2000 CTC.  Instead you will get the non-refundable (up to) $500 Other Dependent Credit.