need help picking up two time pieces

Hello
I have finished the game. however, I am missing two time pieces. I have gone back to find the two missing time pieces but the game will not reveal them to me. I had initially found the timepieces but were no able to pick them up. So I moved on to the next level intending to come back later. Now that I have finished the game. it will not let me reveal the hidden time pieces. So, my question is how do I reveal the time pieces after I get to the level where they are located