Unlocking drop down list in a protected sheet

I unprotect my worksheet and unlock cells that I want users to be able to change. I have a number of drop down list boxes which I unlocked by right clicking each box and choosing Format Control/Protection. Then I protect the worksheet. When I try to change the choice in the drop down box, it does not let me and it says that the cell is protected. The rest of the cells that I unlocked are fine and are updateable.
Can you advise what I need to do to allow the choice in the drop down list to be changed once the sheet is protected?