If you are unable to see any items in the drop-down lists for, say, Activity Items or Expense Items, even though activity and expense items are present in their respective screens, it could be a security permissions limitation. Please check with your CORE administrator.
If there is no change in the assigned security permissions, this might be a data filtering issue. Go to User Preferences and reset your filters. You should be able to see the relevant items in the drop-down lists.
This can also happen if Employee Control is active for you and you have not been assigned any activity or expense items. You need to assign the items to the users before they can see them in the drop-downs. Again check with your CORE administrator. Check CORE Help Center for details.