Shapes are always transparent when previewing

Hi all.

I've just started using captivate and am already encountering a very frustrating problem. I have a photo background and a solid white square at 100% opacity on top. Simple right? Yet when I preview my project its as if the shape is 90% opacity. Same thing if I publish.

I've tried creating a new file from scratch and every single time, no matter what the fill colour is, its transparent when it shouldn't be. I'm losing my mind, please help.