Muffled voice in the back of stage?

Well, the strange thing is.
It is also very obvious when watching movies.
For instance, when I was watching Master and Commander this week, and after the opening scene the crew is speaking to each other on the deck, the camera and mic taking different angles throughout the conversation, from most angles every voice is clear, but in a few cases a voice at once gets very muffled. Not at all comparable to the others, very annoying and destroying the experience my system creates at the moment.

Zd542, thanks for your reply.

Yes of course they do, but still the problem only is there with a certain mic angle.

I have done a lot of experimenting lately with switching gear to find out where the problem lies, but to no avail.

My room is not symmetrical and am a little off the perfect triangle, is just has never been a problem until now.
I am stuck thinking that it is maybe because my stage is a lot deeper these days than it used to be and my room is far from perfect, maybe the most distant sounding sounds will just never be optimal in this setup.