trouble setting up in arch for the oly's

when I set up for snatches or cleans (although its alot worse in snatches) I can’t seem to get my back tight and arched. I have had this problem for months. I have tried so many things, but for some reason I can’t get my chest up and my back tight and arched. This leads me to believe its a flexibility issue, but I am not sure which muscles. Can anyone give me any insight / suggestions on this. btw if you need to look at any workout clips

Try harder. Can you touch your toes lying down on the floor?

yeah, I can touch my toes standing and sitting.