Cottey College, a women’s college in Nevada, Mo., accepted six students from Ethiopia this year, and officials were disappointed when two of them were denied visas. They were puzzled by the reason: The students, the State Department said, did not have strong enough ties to their home country and might not return.

“At the age of 17 or 18?” Megan Corrigan, international education coordinator for the college, said on Wednesday.

Unexpected denials and long delays have become increasingly common for international students and scholars seeking visas, raising concerns among college officials who see a threat to the diversity and enrichment of their campuses, and causing anxiety for students who may have spent years preparing to study in the United States — only to have their hopes dashed.

The latest example to stun educators and students was on Friday, when a Palestinian student, Ismail Ajjawi, was denied entry to the United States. Mr. Ajjawi, who lives in Tyre, Lebanon, was turned back from the airport in Boston after, he said, a Customs and Border Protection agent demanded to see his phone and objected to the social media activity of his friends. He had been set to start his freshman year at Harvard.