Professional :: SWF Works Offline But Not Online?

It plays perfectly well in 'Publish Preview' and when you launch the HTML page offline. However it won't play online. First I published it in Flash 8, where I'd created it, uploaded the swf and the html. In Firefox it loaded a blank page which revealed on right click that there was a Flash movie there, but you couldn't see it or do anything. In IE it just stuck half way through loading. Then I resaved it in Flash CS3 and published it with AC_runactivecontent.js. Uploaded it to my site and the result was exactly the same. To check my syntax a friend uploaded it to his webspace, and put another swf on the same page using the same syntax. The new swf played, but mine didn't. The 'publish settings' for the two were the same. Again, if you look at the page offline both play. My friend made some changes to the FLA and published it successfully on his webspace. He suggested the file size may have been a problem: I had had some fairly large audio files in it and 'updated' them to smaller ones. The resulting swf was 6mb. We wondered if the old ones had been retaines somewhere, boosting the file size.  The only other difference I can see between his version and mine is that there's a font I used which wasn't on his PC. His version has it as Arial (there is only one word in the whole thing). I took his  version, deleted the sound files and reimported the smaller versions, which then published as a 3mb file. But now it doesn't play! Results in IE and FF exactly as above.