3月份入境今天发现I20上没有移民官的盖章

我的也没有盖章，我猜不是必须的吧

http://www.cbp.gov/newsroom/news/spotlights/cbp-i20-stamp

As of Aug. 10, 2012, U.S. Customs and Border Protection (CBP) no longer provides admission stamps on Forms I-20/DS-2019 for prospective and returning international students and scholars (traveling with F, M, and J visas) seeking admission to the United States.

BTW, when I returned to the States in 2011, the CBP officer forgot to write D/S on the admission stamp on my I20 and I had to go back to my local airport (not the one where I entered the US, though) to ask the CBP there to sign it for me.