If you are getting what I've pictured, then I'm reasonably certain that it's because Opera is a Chrome based browser, and it appears to be doing the same thing as Chrome and covering flash elements with the grey 'click to play' box. I don't think there's a solution currently. El miserablo. : (
EDIT : I've just updated to Opera v51 and now the setting in the flash options to 'allow sites to run flash' seems to be working properly. Anyway, to get there it's Menu / Settings / Websites / Flash / Allow sites to run flash.
EDIT 2: Cheers Jim, I already had that set but the box still appeared, but now I've updated Opera, the grey box in Chrome has stopped appearing. It's voodoo, that's what it is, lol.