I have a maintenance plan that's not compressing one of the six databases it's backing up. If I back up that database outside of the maintenance plan it compresses just fine, and I get no errors from the maintenance plan. What could be happening?
What does SELECT @@VERSION show? Maybe there's a bug fix for it if you aren't on the latest SP/CU. Otherwise, you'll need to open a support case with Microsoft.
Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64) Apr 2 2010 15:48:46 Copyright (c) Microsoft Corporation Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1)