Immigration Quotas - Post WWI
When the United States passed the Emergency Immigration Quota Act of 1921 it severly restricted the number of Eastern European immigrants permitted direct entry into the United States. The Act was revised in 1924 but the situation for Eastern Europeans was not much improved by the changes. These laws held until 1952 when the entire immigration system was given an overhaul. However, there was a huge loophole in the 1921 and 1924 Acts in that the restrictions did not apply to immigrants coming from Canada, Newfoundland, and Mexico. Many Eastern European immigrants, especially Jews, immigrated into Canada where they lived for a period of time before continuing on to the United States by train. My Jewish in-laws got here that way, via Halifax. They lived in Toronto for a year before coming to the U.S. (Chicago) via Michigan.
If you are unable to locate U.S. passenger lists for your Eastern European ancestors, take a look at the Canadian and Newfoundland (Newfoundland was separate from Canada at the time) passenger lists and you just might find them. Here is a link to some of the Canadian records.