why I have to wait for

Hi, good afternoon everybody
first My visa is J-I, and then change to be H-1, last year I married to a American citizen. he help me to apply for my green card. my son and I passed the interview May this year, and my son has already received his green card. but they told me I have to wait for a letter , and then bring this letter to stamp my passport, then I will get my green.
I want to know why I have to wait , but my son needn't.
how long time will I wait?
thank you for your help
have a good day