If you are being petitioned (sponsored) to immigrate to the U.S. by a family member or U.S. company or employer, and you belong to a visa category in which the law has placed annual limits on how many can be given out, you might have to wait several years to obtain your immigrant visa or green card. Worse yet, the waiting list that you are on will not always move forward smoothly, but instead might jump, stall, or even reverse course. We'll explain the reversals here.
Although some categories of would-be immigrants to the U.S. are eligible for an immigrant visa or green card as soon as their application is submitted and processed (most notably, immediate relatives of U.S. citizens), many other people only fit into categories within the "preference" system. They are subject to annual limits on "visa numbers," and the numbers often run out.
It's a matter of supply and demand: The more people start the application process to immigrate to the U.S., the longer the waiting list gets.
See Green Card Qualification for a list of categories of preference relatives and employees.
As explained in How Long Is the Wait for Your Priority Date to Become Current?, your place on the waiting list depends on your "priority date." That's either the date that U.S. Citizenship and Immigration Services (USCIS) received the I-130 petition that your family member filed for you or the date the U.S. Department of Labor received the labor certification ("PERM") application that your employer filed for you.
For example, let's say Manuel, an unmarried 24-year old from Mexico, has a U.S. lawful permanent resident father who filed a Form I-130 petition for him on August 31, 2021. USCIS approves the petition, thus making August 31, 2021 Manuel's Priority Date. The first thing Manuel does next is to check the State Department's Visa Bulletin, a list showing "cutoff dates," or which Priority Dates are "current," meaning that their holders are now entitled to visas.
If Manuel had checked the Bulletin for August of 2021, the cutoff date he would have seen in his category (F2B) was "22SEP15," or September 15, 2021. That would have told him that people whose I-130 petitions were submitted around five years before his were only then receiving visas (green cards), and he could expect to wait around the same length of time.
As the months and years go by, and Manuel regularly checks the State Department's monthly Visa Bulletin, he sees the cutoff dates getting closer and closer to his Priority Date. Only when he sees a date that's at least one day later than his, however, can Manual proceed with his green card application, assuming he is still unmarried. (Priority Dates often move forward by seven days at a time.)
At that point, if Manuel were living overseas, he could expect to get notification from the U.S. National Visa Center (NVC) to proceed with processing for an immigrant visa through a U.S. consulate in Mexico. Or, if living in the U.S. lawfully (or otherwise eligible to use the procedure called "Adjustment of Status"), he could go ahead and mail the required paperwork to USCIS, and await a green card interview.
The above example describes the way the visa/green card application process normally works for people in the preference categories. But sometimes, so many people apply after a certain Priority Date is published that the State Department gets overwhelmed, and needs to put on the brakes. It does this by moving the Priority Date in that particular visa category backward. For example, in October of 2021, Manuel might see "30AUG15" as the listed Priority Date.
Manuel's case can go nowhere until his Priority Date moves forward again. His case will be put on hold, and he will need to be patient. Fortunately, this situation should not last long.
If your date is current this month, but you see in the Visa Bulletin that it's due to go backward to a priority date before yours next month, you might, if in the U.S., be able to move forward with your application nonetheless. If you are eligible to use the U.S.-based procedure known as adjustment of status, you can try to do everything possible to submit your adjustment application this month.
If you don't, you'll be back to watching the Visa Bulletin each month to see when your next window of opportunity opens up.
Even though the cutoff date for your category will retrogress next month, your adjustment application will remain pending. USCIS will simply wait until your priority date again is current before it moves ahead on or approves your application.