pdf export problem: pi missing

[Robert Neumann]

Hello,
I am using Lyx 1.2.1 and \usepackage{mathptmx}
I exported my file to a .pdf which worked fine for linux and windows98
But when I tried to open it with a mac, the pi and the infinity symbol were 
gone... There was just white space.
Does anybody know how to solve this problem?
Thanks
Robert Neumann