Saving my fms file

I use an iMac running maverick and the latest v. 4 of FM. How can I regularly save my files? Control +S does not work. Currently, I click on the red x; I get a message asking if I want to save the unsaved media files, I respond “yes”, it saves and the .fms file closes. WHen I reopen the .fms file, everything is there. The process seems to work–usually, but it is a bit unorthodox. Why doesn’t the control+s work?