Please provide some extra details so we can narrow down the possible issues:
1. Double check your journal and post the current status of the entry for the quest. It might be that you did the quest but had to reset and didn't talk to Chea again.
2. Confirm you didn't loot the chest on a previous run through the cavern (might have caused an issue)
3. The item doesn't have a special graphic if I recall correctly, so make sure you aren't overlooking it thinking its a regular skull.
Just trying to eliminate the obvious stuff before David has to look into the code, as there have been issues like this reported before but they are usually easy fixes.