First see if there are any errors in your project. Cause if
there are any errors in your project, Flex Builder will not
generate new SWF. For clearing the cache from the browser, in
Firefox Tools->Clear Private Data and select Cache and the
clear. in IE Tools->Internet Options -> Delete Files.
I ran into the similar problem. I had to include the below
meta tag in the html file to keep the .swf content synced to the
clients all the time. Our users did not like it, but they are
living with it as I could not find an alternative. Not sure why the
flashplayer is not able to recognize the .swf content on the server
changed and bring it down and don't know if this is a browser
limitation (and looks like all have it).