Family Third includes: (F3) Married Sons and Daughters of U.S. Citizens: 23,400, plus any numbers not required by first and second preferences. There are only so many visas permitted in this category. 23,400 is not alot and most of the time this category is oversubscribed. The visa bulletin is a mathematical formula. It is issued by the Department of State each month.
What to do if a Beneficiary has a current priority date?
The priority date is normally the date that the I-130 was filed with Immigration. If the priority date is current, the beneficiary can apply for a green card at the nearest US consulate and enter the US as an immigrant, or if in the United States they can apply for an adjustment of status so long as they are in valid legal non-immgrant status.