It may be an IE9 issue. I found this. The guy mentions a "Workaround", maybe someone having the problem can try it:
ok think I figured this out. It seems the toolbar setting is causing this problem for me. Why it seems to be user specific. If users got it set one way it work another way it doesn't.
Show Standard Editor Toolbar doesn't work
Show Enhanced (WYSIWYG) Editor Toolbar works
This is the workaround I posted at my site.
Settings > General Settings > Miscellaneous Options
Under Message Editor Interface:
Select: Enhanced Interface - Full WYSIWYG Editing