I seem to have a strange structure to my wall. From tapping and using a picture nail, it seems like the structure behind the wall is like this, where the red squares are stiffer and the rest is plasterboard:
I've tried drilling into these stiffer structures and it seems to just be plaster but I meet resistance at about 3cm - possibly metal, I'm not too sure. Any suggestions? My TV mount is a standard flat one, but without a vertical stud, it's hard to know what to do.