Depending on your graphics card, it might be impossible to capture that, outside of some "technical gymnastics".
Probably what's happening, is that the actual content is being offloaded to the GPU to decode and then display in a given space, whether it's a rectangle or the entire screen, overwriting what was already there (often solid black), and the rest of the system really has no idea what actually ends up there. Trying to capture it, gives you what it was supposed to overwrite, because that's what the system actually knows about.
There are several reasons to do that, usually involving performance in some way or another, but it can also be used to make it difficult (or at least non-trivial) to steal copyrighted content. That's a rather poor reason, because a number of GPU's can feed their final result back for the express purpose of capturing like this (performance has no problems being captured), but the mere presence of it can indicate sufficient effort to protect a copyrighted work so that *you* become responsible for stealing it, along with anyone who helps you.
So you're probably not going to get much more help than that. Sorry.