The job market is improving—but not for everyone.

In recent months, employers have stepped up hiring, layoffs have slowed and the unemployment rate has begun to fall more quickly. But the rosier picture hasn't been a boon to everyone without a job. In February, 3.5% of the U.S. work force was unemployed for more than six months, compared with 4.0% in February of 2010, a smaller decline than in the overall jobless rate. The average unemployed worker has been jobless for 40 weeks,...