You will need to make sure that you have all of the tomb keys. My first time through, I forgot the one in the Yeti cave on the main map outside the tomb.
It sounds like you are missing one of the keys from within the tomb. Check all of the passages for doorways and such. If you still don't find it, post a reply with a list of the keys you have and the corridors you have explored and someone can tell you exactly which one you missed and where it is.
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Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.