Intellisense not working for one connection but works for others
Have you installed Visual Studio 2010 SP1?

If so, look at this Microsoft Support article:

FIX: The IntelliSense feature in SSMS 2008 R2 may stop working after you install Visual Studio 2010 SP1

The fix was first released in Cumulative Update package 7 for SQL Server 2008 R2 RTM. It is included in all later updates and service packs. If you have indeed applied Service Pack 3 for SQL Server 2008 R2, you already have this fix.