Adobe Illustrator does not overwrite existing .pdf files

I do a lot of layout/design in illustrator and I'll save each file as .ai and then I'll produce a .pdf version of it for printing, etc. I'm having a continuous problem where I'll make a tweak to a .ai file, save it and then save as .pdf to replace an existing .pdf file of the same name but when I open the newly created .pdf it has all the old content still in place with no updated changes. HELP!