SQL Server 2012, rebuild not lowering avg fragmentation
Is it a small index? Small indexes will always show high levels of fragmentation. You can read more at Ola Hallengren's site which has a highly rated SQL Server maintenance package - look at the PageCountLevel setting, which specifies that Microsoft recommends leaving indexes under 1000 pages - with the link.

http://ola.hallengren.com/sql-server-index-and-statistics-maintenance.html