We saw this same issue a few weeks ago. Went away but it has returned. It appears that the JS that is served to non-Flash browsers (i.e. Mac Safari, iOS) is broken. You can easily see it in the Web console. We isolated the JW script on a blank page so we feel confident it's not us.
[Error] TypeError: undefined is not a function (evaluating 'd')
(anonymous function) (jwplayer.html5.js, line 1)
global code (jwplayer.html5.js, line 2)
Our pages are behind a login so it's not super simple to give an example but I'd assume that this would be easily reproducible on any non-Flash browser.