thats the same type i used on most of my background (see pics in the link in my signature), so i don't think it is a styrofoam issue, i think it is a cement issue what brand did you use? and where in the world do you live? what country?
the type of styrofoam isn't the problem.. you should be using CEMENT.. i would imagine that CONCRETE would be too dry and heavy to stick to the styrofoam.