Have you tried deleting the Program or disconnecting the Program and reconnecting it to GTW?
Josh, Dory, thanks very much for your suggestions.
Josh - Program was already live, with live registrations, so I didn't want to risk losing any info by disconnecting from GTW.
Dory - I did try the refresh option, but it didn't work for me.
Strangely enough, however, Marketo Support tell me they resolved my issue by simply clicking on Refresh from Webinar Provided. I'm unaware why - perhaps something to do with my browser (Chrome), although not sure how.
If it happens again, I'll try a different browser.