Can the priority date go back?
Unfortunately, if you previously filed your I-485 adjustment of status application but your priority date has retrogressed and is no longer current, the U.S. Citizenship and Immigration Services (USCIS) will not adjudicate your case until your priority date becomes current again.
Why is visa Bulletin retrogression?
Visa retrogression is usually caused because the “annual limit for a category or country has been used up or is expected to be used up soon.” Usually, things return to normal (the cut-off dates shift back to where they were before the retrogression) when the new fiscal year begins on October 1.