Can't Embed Anything in Posts

When I try to embed anything in my posts (YouTube videos, the PicApp thumbnail app, etc.), the video or app doesn’t show up in previews or published posts. I’m pasting the codes into the HTML tab as I should, so I’m not sure what the problem is. I’ve tried switching to the “Visual” tab after pasting code in the HTML tab, and the yellow box appears (representing the embedded object). I’m not sure if my theme has anything to do with it (I’m using Oulipo), but I tried switching themes to something else (I forget what) and had the same problem. Or could it be something that’s going on with WordPress 3.2? I can’t determine if the latter is true because I just started self-hosting my site and didn’t use any previous WordPress versions. Does anyone know what I can do to fix this problem? I’m having to go without adding any media! Thanks ??