Find related messages after sending Windows 7 Outlook 2010

Recently we installed Windows 7 Enterprise and Outlook 2010 on all of our computers. On one of them, not all, the sent items are not showing up when I replay and chosse the option: find related bar.
Very important fact: the OST file is 14.5 Giga…… I know it’s a lot but in the same computer with Windows XP it works perfect.
I did few checks: A. rebuild the index B. rebuild the profile C. uncheck the chach mode.
So why are they not showing right away after sending? it’s important also to say that this issue happend 50% works and 50% not works.
Please what ever you can think about I’ll be more than happy to try.
Thanks!