Although all the pieces of the puzzle are in place, the chamber seems to still be waiting for something. It is not reacting properly to the skulls. Perhaps there is something else you need to make the contraption work.
i went and searched and got 4 more of each skull but it still shows the same thing. i finished the first skull puzzle but it wont let me do anything else and i went through all the dialog again still nothing. i pressed the take a step back but it just said find more skulls :/