you really need to provide more information to go on. we do not know what you are connecting in what manner which makes giving any sort of advice hit or miss.
generally though getting only 2.0 on a 5.1 system is one of a few things..
A) you are trying to play a file recorded in stereo not surround
B) your home theater does not accept 5.1 input. this is common on HTIB systems and 5.1 only works on the included dvd player
C) your home theater does not support 5.1 over optical so it is getting stereo input only. while most new systems are fine, old systems may not be.
D) if you run pc-tv-hometheater in chain the pc will only output 2.0