1
general::
Encounter a problem or obstacle. For example, We've hit a snag with this building project. The noun snag has been used in the sense of "a sharp or rough projection," such as would impede passage, since the 1500s.
American Heritage Idioms
2
general::
Phrase(s): hit a snag
Fig. to run into an unexpected problem. • We’ve hit a snag with the building project. • I stopped working on the roof when I hit a snag.
McGrawhill's American Idioms And Phrasal Verbs