like InvertedSpear mentions, check your doc type out, i’ve had problems with IE9 recently and most of it boiled down to the Doc type tags triggering a compatability mode i didn’t need, the same can be true of the meta tags so it might boil down to your Meta tags.
You can always impose a working compatibility mode using the links below too.
from: http://evolpin.wordpress.com/2011/02/25/ie9-compatibility-and-the-meta-tag/
I’ve discovered that this is indeed documented by Microsoft…
http://msdn.microsoft.com/en-us/library/cc288325(VS.85).aspx
“The X-UA-Compatible header is not case sensitive; however, it must appear in the header of the webpage (the HEAD section) before all other elements except for the title element and other meta elements.”